A few (unsubstantiated) guesses at reasons to counter:
1 - Medium risk medium reward opportunities exist (a currently unprofitable but otherwise promising startup addressing a 25m niche can't get bank debt or venture funding)
2 - Could decrease risk holding reward (avg. multiple of investment) constant by having small funds with managers that have deep knowledge of particular niche deploy smaller checks in markets they are better at evaluating (v. large fund managers needing to cast net wider than their "lane")
3 - Size expectation can be correlated with risk. Can decrease risk holding reward (avg multiple of investment) constant by just having smaller funds writing smaller checks for smaller companies that be happy with smaller exits or alt. upside capture (v. forcing companies that could work with small exits into chasing big inflexible exits). Sure no 100x-ers but theoretically could see higher blended fund returns
4 - Opportunities generally continuous on risk and reward calling for equally diverse approaches to capitalization