Luma is four months old. She was 3.5lbs when we got her at 11 weeks. Her sire and dam are both 7lbs. She is still 3.5 lbs at 19 weeks...she has gotten sooo big, height and length wise. But she is so skinny

I never really noticed until now, she got shaved a couple days ago. Is it normal to see all her ribs, and see her spine? I feel around her haunches and there is no fat to grab, just skin.
She was on chicken soup for the puppy lovers soul when we brought her home. We recently switched her to TOTW. She likes this food more it seems. She eats just under a cup of food per day. We give half in the morning, then the rest in the evening. Sometimes she leaves it in the bowl and I have to throw it out.
Shouldn't she have gained weight? She seems like shes tripled in size since we brought her home..but shes skinny as a twig!