I'm sorry to read about Butter's lump, and maybe my experience will be encouraging.

When Lily arrived on our doorstep several years ago, she was sporting a half golfball size lump at the same spot as Butter. The vet determined it was a combination growth/cyst, and after aspirating the cyst twice (it kept returning) Lily underwent surgical removal. Thankfully the growth was not cancerous, and there is no scar or any sign of her surgery at all. So don't worry; since it has decreased in size hopefully it will prove to be nothing serious.