hello and welcome. It is very true that they grow up then out! My yougest is Reese, i believe she is about the height and weight of your girl, and she is almost 8 months. Can you see enders spine? that is when you know they are underweight. At no time should you be able to see the spin orhip bones. if you can i would try feeding a little bit more to see if he can pack on a few more pounds.