You could still produce a AAA title with a small team (e.g 10) in the early to mid-90s. By 2000 an average team was about 100 people. There are exceptions to this in both directions but that was the overall trend.

In that period, you moved from mostly 2D to mostly 3D games so more complex code generally, plus the need for artists to do 3D modelling, texturing, rendering, optimisation, etc etc. Sound designers, composers, etc etc. AAA Games just got bigger, plus multiplayer. Hardware got more powerful (sound and graphics cards) so there was a desire/demand to make more use of that hardware. Generally, it all became a lot more "Hollywood".

There are still great games made by small teams of people in a short period of time. But to hit the AAA mark you need to be with a big studio and have the marketing budget behind you. Again, Hollywood.