I've had two females both spayed before six months. One always looked sleek but she was like that as a kitten. The other one, Pixel, has always had a belly, even when little, about three or four pounds. That belly remains though she is not overweight. She weighs about 11 pounds now. I'm told she may lose weight now that I'm feeding her mostly wet food.
It may be very difficult to tell if spaying changes their personalities--I haven't noticed it. The last girl was mean, and stayed that way until I started letting her outside, then she got a little more relaxed. Pixel's just been a darling the whole time, until recently when we adopted two kittens. Now she is grumpy.
I've always heard things about it changing them, but how, I never really understood. I thought it was a series of myths, but there could be some truth to it so I will keep reading this thread.