It depends on the women.
I had DT's (both 7lbs) and put on a HUGE amount of weight when pregnant because I had SPD and bleeds throughout so was practically bed bound for 9 months. Usually I am a size 8-10 and by the end of pregnancy I was wearing 16-18 and my thighs were huge, I cringe when I see photos of that time. 6 weeks after having them I was back to being 8-10 because I was eating as normal and out and about every day, luckily the SPD went the moment they were out.
My SIL had a singleton pregnancy, she was quite weighty before and also put on a lot of weight during pregnancy, 4 years later she is still carrying the extra weight.
BF is an 8 and she had a simple pregnancy, all she had to do was unbutton her jeans at 9 months, within a week of having her DS her tummy was flat again.
Different women, different outcomes.