How is the chick acting? Is it active and alert, eating and drinking normally? Are you sure it is not the crop filling up?
Does it ever decrease in size with the crop emptying? A leaking air sac from a bump to the chest or side can sometimes be a problem. But more commonly air in the crop may be the problem. If you push on it gently, the chick might burp up some air. Be careful since you could also push crop contents up into the throat and airway, which could cause choking.
For a leaking air sac, also known as subcutaneous emphysema, it is is causing no distress, it can be left alone. If it suddenly gets bigger spreading to the sides, or if the chick is having trouble breathing, you can stick a hypodermic needle just under the skin, and pull the air out with a syringe.