she is a small purebred. when my girl was 6 month and got spayed she weighed 37 lbs. she gained 7 lbs the following week with no exercise and confinement, but she has already lost most of that in just a few days of being allowed to be crazy again.
My male is still only 67 lbs at just shy of 18 months, I don't see anything physically that stands out as wrong on your dog and she looks healthy. I would prefer a slow steady growth anyways She's still growing and will put on weight
She is the same size as my girl and mine is just over six months and she weighs 47lbs. Hope this helps. I notice with her she is growing slow but is healthy. I also notice is she grew taller first then filled in a lot and I think the next step is she's going to grow taller again. At the size her parents were I don't care if it takes 24 months to finish growing as long as its a slow steady pace and she is healthy. (Mom 85lbs and dad 97lbs)