People often suspect something is wrong with a chick or chicken if their crop is full. In the case of chicks or juvenile birds this is usually normal as they will eat quite a bit during this fast growth period. It often happens with full grown chickens as well and is usually normal as long as they all have the same sized bulge. You shouldn't worry unless one chicken has a much larger crop than the rest, or the chicken is acting "funny" indicating distress.
If the crop is "hard" to the touch, the crop could be impacted. This happens when feed is "stuck" in the crop and the chicken can't "swallow" or pass the feed on down to the proventriculous and gizzard.
How it happens:
The crop may occasionally become impacted when feed is withheld prior to worming, causing chickens to eat too much afterwards. Crops may also get packed when birds are free ranged where little is available to eat but tough, fibrous vegetation. Even if the bird continues to eat nutrition cannot get through. The swollen crop may cut off the windpipe, suffocating the bird. Crop impaction is not likely to occur in properly fed birds.
In mature birds: distended, sour-smelling crop filled with feed and roughage. The crop feels hard when pressed between fingers. Emaciation.
Provide proper rations and plenty of clean, fresh water. If feed is withheld prior to worming, offer a moistened ration 1 hour after worming.
Gail Damerow's "Chicken Health Handbook" lists the treatment as follows:
Disinfect skin, slit through skin with very sharp blade, pull skin aside and slit through crop, clean out crop, isolate bird and keep wound clean until it heals.
I have been raising poultry for over 30 years and could not bring myself to get that drastic. Surgery (which this is) is serious and best left to professionals. We have never had this problem with any of our birds but have found the following treatment in several references and would certainly try it before resorting to surgery which should be done by a veterinarian.
If you suspect impacted crop, administer two eye-droppers full of mineral oil (NOT MINERAL SPIRITS!) by forcing an eye-dropper down the chickens gullet, and massage the hard lump between your fingers to soften it up and help her pass it. Repeat if necessary the following day. If the crop remains hard and the chicken appears distressed, then you may resort to the above treatment or take it to a vet.