There's probably some "hybrid vigor" going on - notice that Rainbow's dad has the same body build type as Big Mac. This is also true of Rarity's dad, Shining Armor, Fancy Pants and the Royal Guards in general.



My personal theory is that over the many centuries of Equestria's existence there developed well-mixed lineages of earth-pony/unicorn and earth-pony/pegasi hybrids, most of them being upper-class members and having advantages in strength and endurance compared to "pureblood" unicorns and pegasi. Twilight and her brother, Rainbow Dash and probably Rarity and her sister all have benefited from this kind of mixed blood in their family lines.