A unicorns buffness is based on how much magic they use in comparison to how much they use their bodies.
The more weighted they are towards magic, the slimmer they are. And vice versa.
Most unicorns have a balance, like unicorns in Ponyville and Twilight to some degree.
This is all speculation of course.
Blueblood is fit because he spends hours posing for himself in front of a mirror.
Fancypants likes gardening.
Being critical is a burden which some people loves to bear.
PC's back in business!
Ended up being the graphic's card that was borked, not the PSU. D: