NO they will be fine, they will run off most of there fat. I haven't heard of a duck that eat to much and now it can't walk, not to say it's not possible, I just haven't heard of it yet..
Nannn let them eat what they want and however much they want with being outside they will move around a lot and run off all that fluffiness sort of speaking.
Mine go in a dog crate at night without a heating lamp, once the adult feathers start to coming, no need to worry to much about them getting cold. there's quite a few there so they will cuddle up with each other to keep warm. I think they will be OK. at three weeks they do get messy and...
OMG, SWIM TIME...
Let them eat all they want.
I feed mine All natural layer crumbles just check the calcuim in it, My feed store quit selling All Flock... But a gamebird or meatbird feed will work too.
give her time, she'll BLOSSUM, she's just got a slow start but in a few weeks BOOM, she'll catch up. LOL, I had a call duck very small, that hatched and sure enough, she's the same size as my other four that hatched with her.