I wonder if their human teeth wear out faster, trying to satiate their horse-sized appetites.
I'd also think that their lungs and heart, at least, would have to be horse-sized in order to support a horse-sized metabolism. There are human beings who weigh as much as a centaur might, but I doubt that even in the morbidly obese there's much growth with respect to their internal organs.
But then they don't run 35 mph either.