Sorry, I phrased that incorrectly/poorly. What I meant to say is that market cap weighting allocates more money to stocks that have a higher market cap (and the stocks got that way by going up in the past), pushing up the price even more.

This wouldn't be true if all shares of a company were all trading at the same time, but that's not the case. The number of available shares is more constrained than the total free floating market cap would suggest.