She sounds like a little sweetheart.
Please, please remember that your granddaughter, being so young and tiny, shouldn't be eating a lot at her age. Her stomach is only the size of her fist. 1 1/2 to 2 ounces is a lot of formula/pumped breastmilk to take in. If she has reflux that's being treated, she'll keep more of it down. She also should still only be eating between every 2 or 3 hours. Feed her whenever she starts to show cues and signs of hunger.
As long as the baby appears happy and is gaining weight, and is hitting most of her typical milestones she should be fine. I would listen to the doctors and those trying to reassure you. You don't want to overfeed the baby, because you would be teaching her to ignore her own body's signals letting her know when she feels full and that would NOT be a good situation.
EDIT: I thought I mentioned this, but I agree that it's imperative to see a Pediatric Gastroenterologist. They can best treat your tiny granddaughter. And please have your granddaughter's mommy see her primary care physician so that she can get a referral to a psychiatrist to deal with Post Partum Depression. She doesn't have to cope with PPD alone. Take it from a mom who wishes she had received treatment much, much sooner than she did.