Does she look and feel good? You really need to put your hands on her. You should be able to not see ribs, but feel them under a little layer of fat (you don't want a comforter or a sheet of fat, but a light duvet is ideal).
Luna was about the same at 6 months and seems to have topped off at 60 pounds. She's been 60 pounds since she was a little under a year old.