It looks a little odd, have you had a feel gently? is it just general swelling along the leg, or possibly a large bump or abscess on it? It looks more like a lump, than swelling, but it's hard to judge from a photo.
If the leg is that swollen, I imagine it's possible that the bone is fractured, is there any possibility the chick could have fallen from a height?
Is it still walking? Or just laying down?
It may be a good idea to separate the chick, and confine her to a smaller space so she is inclined to rest.
Is a vet an option? They would be able to diagnose the situation, and if it is an injury to the leg, they could prescribe medicine to help with pain and inflammation.