Chickens of all ages are good at conspiring to worry us with strange symptoms, but young chicks are experts in this arena. Thankfully, most chicks throwing these worries at us manage to resolve them and never become sick from them.
I had a new chick many years ago that was pooping practically straight water from day one. It was very loud, too, so I couldn't pretend everything was just fine. The little thing was just as active as the others, never slowing down as one would expect a chick to do that was sick with something. Her watery poop, from close observation was likely caused by an obsession with drinking water. This was a quirk that she never grew out of, and she died at age eight years never having been sick a single day in all that time.
I would offer this explanation for your chick's strange crop appearance - that it is due to very thin down in that area combined with a tendency to pig out. Some chicks are as obsessed with food as my chick was with drinking water. While my chick kept her strange watery poop her entire life, I predict your chick will appear more normal after she grows her first feathers in and they cover her strangely ballooning crop. Also, the crop will become less conspicuous as her size increases.
But do keep monitoring the chick's behavior. If this is an air sac issue, eventually it would affect her behavior if it's due to airsaculitis. She would become lethargic and then something would need to be done about that.
