I like the interpretation of Cadance versus Twilight here, where Twilight is stick thin from a combination of sparse eating due to her constant studying and other librarian pursuits as well as her alicorn magic burning what she does consume so readily she never has a chance to put on weight.
 
Meanwhile Cadance has thunder thighs and a full figure, which stems at least partially from the food she’s probably used to eating. I like to think Cadance took too much of a liking to rich food once she ascended, so her more sedentary lifestyle and her usual royal duties lend to her being a bit chunkier. It’s not in any bad way at all as she’s still healthy, just larger than Twilight.
 
Yes all of this is quite stylized with the art style–which by the way I quite like–but when you extrapolate the art style into realism you can see what I’m saying.