How do you get more of the food you eat when pregnant to your baby instead of so much to you.?

Why is it that some women can stay thin while pregnant and have big plump babies and some woman can get really fat when pregnant but still have a small baby, I would like to know if there are any certain foods, or exercises or something to get more of the food and nutrition (and fat)you ingest to your baby instead of so much to your body.