I was 7 stone 12 when I became pregnant with ds1. despite hyperemesis and swimming three times a week, (once I felt better) and eating a normal diet, I went up to 11 stone - so - oooh! I gained 3 stone 2lb! My body seemed to cling onto every calorie that it could - righteously so. I felt slightly panicky, and deffo didn't overeat - but it was slightly unnerving
After ds1, I breastfed, and lost weight etc etc though tbh - although BF does help you lose weight, personally, I found that my body clung onto those last few pounds whilst I was breast-feeding, as some kind of reserve, and when I finished, a few weeks later, I was back at 7 stone 12. It took me about a year.
Cue Ds2 - when I started I was 7 and a half stone. Again, my body needed the reserves, and I went up to 11 stone - despite eating normally, swimming etc This time, I didn't have the opportunity to breastfeed, having septicaemia following ds2 - but a year later, was back to 7 and a half stone - which is where I am now still (they are 14, and 8)......
So yeah - I put on a lot in pregnancy proportionally, despite regular exercise and eating my normal stuff - my body needed to, and I let it. I lost it again afterwards without dieting, and going back to normal exercise. It wasn't my priority though, losing the weight - it was just that exercising again, made me feel more like me. And I certainly didn't snap into shape - pmsl! I was narky, grumpy and still looking pregnant for a good few months afterwards!
Everyone is different....