Advice on my round shaped runt

Please meet the mighty Meatball! Named for her perfect meatball shape. The issue is that she doesn't seem to be growing. The others are all doing great but this one constantly has poop stuck to her fluff (not pasty butt) and doesn't seem nearly as stable on its feet as the others. It's still eating/drinking and scratching around. They're just under two weeks old. She seems especially rotund and I can't tell if it's the breed or a deformity, the back looks almost hunched.

Has anyone had a chick that looked like this? I got a mix of breeds plus a freebie so I'm not 100% on what kind it is.