It hasn't evolved as much as some people think. You take a prime Sak, Arona, Fedor, or Hendo and drop them in the UFC right now and they will contend, at the very least. Fighters do seem bigger now due to advances in diet and weight-cutting, but skills-wise there's a trade-off involved in being more "well-rounded". Exceptions are Israel and Khabib who are great at one thing, and it has worked out well for them so far.