Well, in my opinion, Twilight and Cadance are from different breeds though they're now alicorns. Being a princess, specifically a Crystal pony princess, makes Cadance's design being made to make her look sleek and graceful (get it.... crystal + princess + grace, kinda like the swan story, about the graceful ballerina).
In which, Twilight is more bulkier (not sure if this is the exact word to be used for her but you get the idea, I hope) than Cadance because she's not an alicorn which belongs to the Crystal Empire.
Edited post to remove the link because I have inserted the wrong movie U