The Size Factor: Academic Evidence, Post-Publication Decay, and Practical Implementation

The size premium — the historical tendency of small-capitalisation stocks to deliver higher long-run returns than large-capitalisation stocks — is one of the foundational findings of empirical asset pricing. It was documented by Eugene Fama and Kenneth French in their landmark 1992 paper on the cross-section of expected stock returns, and the following year (1993) they formalised the three-factor model that extended the Capital Asset Pricing Model by adding size (SMB: small minus big) and value (HML: high minus low book-to-market ratio) as systematic risk factors alongside market beta.

Understanding the size factor — what the evidence actually shows, why the premium might or might not persist, and how to implement small-cap exposure intelligently — is valuable for investors constructing globally diversified portfolios.

The Original Evidence

Fama and French's 1992 study examined US stock returns from 1963 to 1990. They found that small-cap stocks — those in the lowest quintile of market capitalisation — delivered substantially higher average returns than large-cap stocks over the sample period, even after controlling for market beta. The annualised size premium in the original sample was substantial: approximately 3–4 percentage points per year in raw terms.

The finding was reinforced by subsequent work extending the analysis to earlier data and international markets. Dimson, Marsh, and Staunton's Global Investment Returns Yearbook, which covers equity markets in 35 countries going back over 120 years in some cases, consistently documents a positive size premium in long historical samples for most developed markets.

The theoretical explanations for why a size premium should exist are plausible:

Illiquidity. Small-cap stocks are less liquid than large-caps. Institutional investors cannot easily take large positions without moving the price. This illiquidity is a genuine economic cost that investors should expect to be compensated for via higher expected returns.

Lower analyst coverage. Small-cap stocks are followed by fewer analysts, meaning information is less efficiently incorporated into prices. Skilled investors who do their own research can potentially exploit genuine information asymmetries in this space in a way that is not possible in large, efficiently priced stocks.

Higher distress risk. Smaller companies have less diversified revenue bases, less access to capital markets, and higher failure rates. The size premium, on this view, partially reflects a genuine risk of financial distress rather than a free lunch.

Post-Publication Evidence: A More Complicated Picture

Academic finance has observed repeatedly that factor premia documented in published papers tend to be smaller — sometimes dramatically so — in the period following publication. This phenomenon, known as "factor decay" or "publication bias", reflects a combination of genuine arbitrage (investors exploit the premium, compressing it), data mining (some apparent premia are artefacts of the specific sample examined), and specification issues.

The size factor has been one of the more contested examples. Several studies examining the US market post-1992 find that the raw size premium is smaller than in the original sample and, crucially, becomes statistically insignificant after controlling for the quality of the underlying companies.

The "size premium without junk" argument, articulated by Clifford Asness and colleagues at AQR Capital, is particularly important. They find that much of what appeared to be a size premium was actually a quality-minus-junk effect embedded in small-cap stocks: many small companies are unprofitable, financially fragile, and heavily reliant on sentiment. When small-cap exposure is restricted to companies that are also profitable and financially stable — "quality small-caps" — the size premium is more robust and more consistent across time periods and geographies.

This is a meaningful insight for implementation: not all small-cap exposure is equivalent.

International Evidence

The picture is more mixed internationally than in US data. Some markets show persistent and statistically significant size premia; others do not. UK AIM (Alternative Investment Market) stocks, for example, have exhibited high volatility and inconsistent returns relative to FTSE Small Cap companies, partially reflecting the concentration of early-stage, loss-making businesses on AIM.

The FTSE Small Cap index in the UK has, over certain periods, demonstrated genuine return enhancement relative to the FTSE 100 — particularly in the early stages of economic recovery cycles, when risk appetite is recovering and smaller domestic businesses benefit disproportionately from improving conditions. However, in periods of market stress, small-caps typically fall more sharply and recover more slowly than large-caps, reflecting their higher operational and financial risk.

The Cyclicality of Small-Cap Performance

Small-cap stocks tend to exhibit a consistent pattern relative to the economic cycle. They typically:

• Underperform during recessions and market stress. Tighter credit conditions disproportionately affect smaller companies; institutional risk aversion flows towards large liquid names.
• Outperform in early recovery phases. As credit loosens and animal spirits return, smaller companies with higher operational leverage benefit more than large-caps from expanding economic conditions.

This cyclicality suggests that tactical tilts towards small-cap exposure — increasing allocation early in recovery cycles — have some empirical support. However, timing cycle phases accurately is extremely difficult in practice, and most academic evidence favours strategic (buy-and-hold) factor exposure over tactical factor timing.

The Profitability Interaction

As noted above, the quality interaction is crucial. The combination of small size and high profitability (strong return on equity, strong gross profit margin) produces more consistent and robust historical outperformance than small size alone.

This is consistent with the theoretical argument that unprofitable small companies represent genuine distress risk rather than a compensated factor premium. By contrast, profitable small companies that are ignored by large institutional investors due to capacity constraints and liquidity requirements represent a potentially persistent source of excess return.

Several fund managers and factor-based ETFs have built strategies explicitly targeting the intersection of small-cap and quality, variously described as "quality small-cap", "small-cap growth", or "profitable small-cap" strategies.

Implementation: ETFs and Funds

Practical implementation of global small-cap exposure has become straightforward through low-cost ETFs:

iShares MSCI World Small Cap UCITS ETF provides broad exposure to small-cap stocks in developed markets with an ongoing charge of around 0.35% annually. This captures approximately 4,000 smaller companies across the MSCI World universe.

Vanguard FTSE Global All Cap UCITS ETF includes both large, mid, and small-cap stocks globally — effectively a genuine all-cap portfolio — at an ongoing charge of approximately 0.23%.

SPDR MSCI World Small Cap UCITS ETF provides a similar developed-market small-cap exposure with competitive pricing.

For UK-specific exposure, the HSBC FTSE 250 UCITS ETF provides mid-cap UK exposure (the FTSE 250 is predominantly UK-focused domestic companies and may not represent the global size factor), while the iShares MSCI UK Small Cap UCITS ETF covers the smaller segment.

A strategic allocation of 10–20% of the equity component to global small-cap, combined with a core developed-market large-cap index, represents a reasonable practical implementation of a size tilt for a long-term investor comfortable with additional short-term volatility.

Position Sizing and Risk Management

Small-cap equity allocations should be sized with the understanding that they carry higher volatility, higher drawdown potential in market stress events, and typically wider bid-offer spreads than large-cap equivalents. For investors with shorter time horizons or meaningful near-term liquidity requirements, a smaller or zero small-cap allocation is appropriate.

For long-term investors (15+ year time horizons) with no near-term liquidity demands from the equity portion of their portfolio, a modest strategic small-cap tilt — implemented via low-cost diversified ETFs rather than concentrated single-stock bets — is consistent with the academic evidence on long-run factor premia.

Compliance and Regulatory Note

Investments in smaller companies typically carry higher risks than investments in larger, more established companies, including greater volatility, lower liquidity, and higher risk of business failure. Past performance of specific factors or market segments is not a reliable indicator of future returns. All investments can fall as well as rise in value. This article is for information only and does not constitute personal financial advice. Tax treatment depends on individual circumstances and applicable law.

How Global Investments Can Help

Factor investing — including small-cap exposure — is an area where implementation precision matters considerably. The difference between a well-constructed small-cap allocation (diversified, quality-screened, cost-efficient) and a poorly constructed one (concentrated, opaque, high-cost) is material over a 10-year holding period. At Global Investments, we assess each client's total equity exposure, current factor tilts (often implicit rather than explicit), and risk tolerance before recommending any specific factor allocation. Our approach to portfolio construction is evidence-based and grounded in academic research, adapted to the practical realities of international investors. If you would like to explore whether a small-cap or broader multi-factor tilt is appropriate for your portfolio, please contact our team.