I think there is a portion you can control, and a portion which is just how your body manages pregnancy.
I gained 13kg with my son, mostly bump but my arms and legs did grow. Pretty sure 3kg of that was from slack eating, but I lost most (11kg) within 10 months.
A fairly petite friend of mine put on 30kg with her son. And I swear she didn't overdue it. Lost it all within 12 months.
I'm very early with my second so I'll let you know how I go. I've been much more active since having DS, so I'm hoping to stay around 10kgs.