help?!

My 5 month old only weighs maybe about 13 pounds now. So a lot of people think I'm crazy since a lot of babies her age are pushing 16 pounds or better. She just doesn't eat that much. She only eats about 2-4oz a feeding and when I give her baby food she eats about half the container. I feel like she's not eating the way a 5 month old should. Is this normal or should I try getting her to eat more? Thanks for any advice ladies!