I have noticed that Andy's butt is bigger this year. I LIKE THAT!
If you think about it: Serena, James, Andy, Andre have some serious booty. So does Rainer too. I myself have a big butt too and can tell you that my legs are very fast and very strong kinda like a male Kim Clijsters build (who has some booty too). I think it makes your ability to balance yourself, absord and generate power both and move explosively.