Skinny babies

One of my MILs friend visited our house last week to see our baby, I was quite offended when she made a comment that my baby was so "skinny", as a mom it kinda made me think if I'm feeding her well enough. Bub was healthy but a small baby when she was born and every TCU I always check with the doctor if her BMI is normal. She's gaining normally though. Anyone facing the same situation here?