This was in Canada rather than the US, but I know that one of my mom's GP's concerns about her rapid weight gain during the earlier stages of her pregnancy (my dad took the "eating for two" phrase a bit too literally and pushed her to eat large portions of high-fat foods) was that she'd end up having to birth a 9-10 pound baby, along with other health worries. To be fair, however, my mom is quite tiny (small frame, small hips, barely 5' tall), and it wasn't necessarily that he wanted her to have a small baby to make the birth "easier" on her so much as concerns that she'd have an oversized baby and wind up having complications.