For starters, not all women respond to pregnancy the same weight wise. Just cause a chick has a baby doesnt mean shes gonna blow up. A lot of weight gain over time is just due to metabolism slowing down.
Secondly, some dudes like a "one cheeseburger away" chick. Why would such a dude impregnate a skinny chick on the off chance that pregnancy would make her thick? What happens if he puts a baby in her and she emerges from the pregnancy w/no weight gain?
U nikkas simultaneously think too much and dont think enough. Its not hard to maintain weight, even with a baby. Stay active, eat right, dont let shyt get out of control.