My first dog had a similar lump on her hind leg the size of an egg. We had a needle biopsy done and they said it was benign not to worry. It continued to grow to the size of a baseball. We had it removed because even though it did not bother her, it did not look nice. She never had a problem after that with her leg. I suggest taking your dog to a Vet just to be sure it is not cancerous.
