Is it 600 pounds though? Centaurs aren't the size of a horse, they're smaller. Additionally their internal body structure is different. Certainly lighter as you don't need a collosal intestine system to turn grass into energy, and that's before you ask 'hang on what's the heart/lung situation we got going on here.
We can't compare just horse+human to establish what a centaur could or could not do. They're unrealistic beings by nature, so applying small bits of realism to them is just going to result in the whole realism house collapsing on 'em.