The way I've always interpreted weapon sizes, the reason for which you can't use a Huge greatsword even if you've got a 32 Strength is one of mass. Regardless of your magically-enhanced Strength, you can't use a Huge greatsword because, well, the sword would swing you. I know that you can carry it easily, but in combat you would be unable to stay balanced, and the harder you swing, the harder it would be. I don't know if this is actually a reasonable explanation, but it sounds good enough.
In this view, a centaur could easily use bigger weapons, because it weighs a lot more than a human, in addition to being strong enough.