I don't think weight gain has anything to do with baby size. i gained a LOT of weight with DS, didn't weight myself at the end but i would guess it was about 60 pounds. He was born 6.9 pounds. my mom also gained 60 pounds with my sister who was 6.3 pounds and only 30 lbs with me, and i was 7.14 ounces.
even though small babies run in my and DH's family, i was surprised DS was only 6.9. but looking back, even though i gained a lot, i did not have a big appetite at all. in the first trimester i probably didn't eat enough!