Bigger is usually better... but it depends
I've been on 2 cruises so far, one on MTS Triton, a small no name ship (22 000 tons) and on the Carnival Ecstacy (about 70 000 tons).
On the triton there was great entertainement and a great sense of community. With only 700 people on board, you often ran into the same people and friendships were formed. On the Ecstacy we had more fun cause it was bigger and there was more to do. The food was also better. But you don't have the same sense of community. So yah, bigger is better. I'm going on the Explorer of the seas in December, a 130 000 ton ship... it's BIG! I hope it'll be fun.