I don't want to come across as rude when I say this, but you are wrong. Cadence is the same size as the large stallions with visible hooves which means she both bigger than the large mares with the long necks like Sassy Saddles and smaller than Luna. Which means that in human terms Cadence is something like an amazon in build while Luna and Celestia are more like giants. True they have show two ponies who don't fall in the normal size range who aren't alicorns, Trouble Shoes(who a little bigger than Celestia) and Whoa Nelly who looks to be as tall as Luna, but they could just have pony gigantism. Would explain a lot about them.