Size Factor Investing: The Small-Cap Advantage
Small-cap stocks have historically outperformed large-cap stocks by 1-3% annually. The size premium is strongest among the smallest companies and is enhanced when combined with value. AVUV (small-cap value) has returned over 12% annualized since 2019.
Size factor investing targets the historical tendency for smaller companies to generate higher returns than larger companies. This small-cap premium was documented by Rolf Banz (1981) and incorporated into the Fama-French three-factor model. The logic is that smaller companies are riskier — they have less access to capital markets, higher business failure rates, less diversified revenue streams, and less analyst coverage. This higher risk commands a higher expected return. The size premium has been observed across international markets and most time periods, though it is not as consistent as the value premium.
The size premium is most pronounced among the smallest companies (micro-caps, under $300 million market cap) and when combined with value (small-cap value). A dollar invested in the smallest decile of NYSE stocks in 1926 would have grown to over $40,000 by 2020, compared to approximately $5,000 for large-cap stocks. However, this premium comes with significant volatility and extended periods of underperformance. Small caps underperformed large caps from 1979-1990 and again from 2014-2020. These periods test investors' patience, making the size premium a risk premium that requires the fortitude to hold through underperformance.
Real-world example: An investor who allocated $10,000 to IJR (iShares S&P Small-Cap 600, tracking small-cap stocks) and $10,000 to SPY in 2000 would have seen IJR turn into $52,000 by 2025 (7.5% annualized) versus $48,000 for SPY (6.9% annualized). The small-cap advantage was driven by stronger performance during recoveries — small caps gained 48% in 2003 vs 28% for large caps, and 31% in 2013 vs 32% for large caps. Small caps also suffer more during downturns: in 2008, IJR fell -37% vs -37% for SPY (surprisingly similar), but in 2020, IJR fell -33% vs -34% for SPY. The size premium works over decades, not years. The combination of size and value is even more powerful: AVUV (Avantis US Small Cap Value) returned approximately 12.5% annualized from inception (2019) through 2025. Value factor investing →
Implementing the Size Factor with ETFs
Several ETF options provide size factor exposure. For broad small-cap: VB (Vanguard Small-Cap, 0.05% ER) covers the CRSP US Small Cap Index. IJR (iShares Core S&P Small-Cap 600, 0.06%) covers the S&P 600. AVUV (Avantis US Small Cap Value, 0.25%) combines size and value with profitability screens. DFSV (Dimensional US Small Cap Value, 0.31%) is similar but only available through advisors. For micro-cap exposure: IWC (iShares Micro-Cap, 0.60%) covers companies under $300 million. For international small-cap: AVDV (Avantis International Small Cap Value, 0.36%), SCHC (Schwab International Small-Cap Equity, 0.12%). The optimal size exposure for most investors is a 10-20% allocation to small-cap value (AVUV or DFSV) combined with broad market funds (VTI/VXUS). This provides size exposure at the most attractive intersection of value and size without overconcentrating in micro-caps. The size premium is more tax-efficient than momentum but less efficient than buy-and-hold market indexing due to higher turnover in small-cap funds.
FAQs
Is the small-cap premium dead?
The small-cap premium was declared dead many times, only to re-emerge. From 1979-1990, small caps underperformed large caps, leading many to conclude the premium had disappeared. Then small caps outperformed from 1991-2013. From 2014-2020, small caps again underperformed, leading to renewed debate. However, small caps have historically outperformed over the very long term (1926-present), and the premium persists internationally. The recent underperformance is partly due to the dominance of mega-cap tech stocks and the shift from active to passive investing. However, the risk arguments for the size premium remain valid — small companies are inherently riskier and should command higher expected returns. Investors should expect the size premium to continue but with significant variability.
What is the difference between IJR, AVUV, and VB?
VB (Vanguard Small-Cap) tracks the CRSP US Small Cap Index, covering the bottom 10-15% of US market capitalization with no value/growth tilt. IJR tracks the S&P 600, which requires companies to have positive earnings — a profitability screen that has historically added 1-2% annualized returns. AVUV tracks the Avantis US Small Cap Value Index, which targets small-cap value stocks using multiple value metrics (P/B, P/E, P/CF) combined with profitability and investment screens. AVUV has the strongest factor exposure (deepest value, smallest companies) and the highest expected premium but also the highest fees (0.25%). For most investors, AVUV provides the best pure size+value factor exposure, while IJR provides a more moderate small-cap tilt with a profitability screen.
How much of my portfolio should be in small caps?
The market weight of small caps (bottom 15% by market cap) is approximately 10-15% of total US market. A moderate small-cap tilt would allocate 15-25% of US equity to small-cap funds. An aggressive tilt would allocate 30-50% to small caps. The optimal allocation depends on your tracking error tolerance and factor conviction. Most factor-aware investors allocate 10-20% of total equity to small-cap value (AVUV), which provides meaningful factor exposure while maintaining reasonable tracking error versus the broad market. Beyond 30% of equity in small caps, tracking error becomes significant (5-10% annually), which can be difficult to maintain during periods of small-cap underperformance.