aaand here we go with the pegasi being able to fly and stuff with hollow bones and stuff
people, we're talking about a cartoon show here, it rained chocolate milk from cotton candy clouds so trying to apply science to the show doesn't really work.
i've explained this many times before, it's simply magic.
before you go about saying "but only unicorns can use magic"
sure they're the only ones that can harness it's power and use it to do certain deeds, but that doesn't mean it only revolves around them, it's everywhere, it can pretty much explain everything without further thought - why rainbow dash can fly, for example. and it somehow probably gives earth ponies an extra "boost" to make up for not having a horn or wings. pinkie pie's defying logic and exceptional energy or applejack's strength for example.