coder's ( new abilities/adjustments) artists (mobs/gear) animators, effects artists, writers, internal testing, just to name a few of the basics. a lot is recycled, but it all has to go through different parts of the team. there is also the overhead people, and localization which isnt always concidered, but actually contributes to development. as well as the various directors (battle, quest, systems) and finally the producer.

if you count the directors into doing multiple parts covering some code, and doubling as writers, that's still much much more work than a 6 man team could feasibly carry.. doing 4 major/3 minor updates a year. putting 2-3 people per area, leads to between 20 and 30 people. i didn't list all of the jobs required, just the ones involved per patch off the top of my head.