I know everybody here loves VTSAX. As a practical matter, my 401k's only decent option is an S&P500 index, so that gets a significant chunk of my savings. The logical next step would be to balance this out with small caps in the IRA, HSA, etc. But, when I look up what VTSAX is composed of, the percentages seem somewhat arbitrary and I'm wondering if anybody has a good answer as to why the VTSAX weighting of large, mid, small cap holdings is considered optimal and some other weighting of S&P500, S&P600, etc. is inferior (ignore differences in expenses). Why not more small and less mid for example?