VTSAX and VFIAX overlap about 80-85%. They both own the S&P500. VTSAX also owns the smallest (by market cap) 10-20% of the market. So that's the difference between the two. Note that although the difference seems large by number of ticker symbols, it's not that big in dollar terms since the smallest companies in the stock market are much smaller than the largest companies in the stock market.
On any given day, small caps can outperform or underperform midcaps and large caps, and you should see VTSAX outperform or underperform VFIAX in a similar fashion. The only difference is that I think the dividend dates are a few days off between the two funds, so you might see some differences in expected behavior around the last few days of March, June, September and December when that happens.
Long term, the two funds should perform pretty much exactly the same.