Baby eating less with AR formula?

I just switched my LO to the enfamil AR bc she has silent reflux, but I've noticed shes eating less, like it will vary from 3.25 oz to only 1.5 oz. We try to get her to eat more but she either asleep or doesn't want to eat more. She's 7 pounds now at a month old and was 6.10 pounds at birth. I'm wondering if anyone else has noticed their baby eating less with thinker formula?