I don't think it the crop, too low. It almost seems like a hernia, or something along those lines. It can be reduced if pressed on. Other than this the little one is eating, playing, sleeping...... all the normal things.
"Blister-like lesions, called breast blisters, can develop on the keel bone area of the breast. They may be difficult to see under the feather cover but can be detected by blowing the feathers away from the breast to reveal the underlying skin. Maintaining dry litter condition, preventing overcrowding, using equipment without sharp edges, and using feeding programs that develop good body structure before heavy weights are reached, are all factors that can help reduce the incidence of breast blisters." from http://edis.ifas.ufl.edu/PS035
Does she sleep on the ground? Lay on wire a lot? Rub on anything sharp or hard? According to my chicken health book, it's should resolve on it's own, but if it becomes infected you'll have to lance, drain, and clean it. That doesn't look infected; infected would be crusty and red or leaking colored pus.
I'm still going with breast blister. They can form on the sides, and my chicken had a huge one that took up all of her breast on the left side. But whatever it is, it's most likely falls under the category of Not A Big Deal.