I've just been reading another post - Sensei and Size, and it's rather interesting.
So I thought, does being heavier (larger centre) improve one's Aikido?
Due to greater ability to keep weight underside and being more balanced/stable.
Actually, I think that's why a lot of aikido and judo people get heavier--so they'll be harder for the opponent to move.
On the other hand, the heavier one gets, the harder it is to move him/herself. After all, every time you get thrown, you have to get back up...
And every time you go to a knee, you have to stand back up...
And every time you simply do irimi/tenkan, etc., you're moving extra weight around.
There's definitely an ideal point for every person. Like an airplane, thrust is offset by drag. So you can only benefit by extra heaviness up to a point.