In my head!canon, Millicent would more closely resemble a girl who plays rugby rather than a girl who is a figure skater or a ballerina just as far as body type goes. I wouldn't say fat, just larger than the average girl with good muscle mass. That being said, I don't ever picture Millicent as ugly because I am sceptical of Gryffindor descriptions of their less-liked Slytherin counterparts. I just assume there is a fair amount of bias there.