Think of all the women in the 3rd world countries with very little food and they manage to have normal size babies and my cousin gets that real bad morning sickness where she has been hospitalised for dehydration and put on a drip many times and cant keep anything down the whole pregnancy and she is really skinny before pregnancy and never gains a thing but loses weight from being so sick and all 3 of her kids were born normal 7-8lb babies.
Try not to worry to much i only have a small belly and all my bubs have measured 2-3 weeks behind and had little heads 33-35cm but all were between 5lb and 7lb and healthy womens bodies are designed to look after bubs first we get the leftovers.