Pinkie Pie eats a crud loud though, I'm surprised she doesn't have diabetes XD. And even if Applejack is much stronger in the legs, she would still have to carry the apple barrels with her hands, I mean, there is no straps on them, unless they are a bit lazier, and it's obvious AJ is not lazy. But if she DID carry the apple barrel on her back, she would have to hold on to the sides. The only reason Pinkie can do her crazy stuff is cause she defies logic, but that wouldn't mean she can defy her body build, cause I mean...that is just impossible. Not even the great lengths of Pinkie is capable. Think about it.
A farmer that works all day VS a logic defying girl who eats a lot.