Smaller people tend to put on more weight than larger people for some reason.
I put on 20kg with my first pregnancy and I am tiny! But I ended up having a big bub, and also mild polyhydramnious. I was 47kg pre preg #1 and without really trying after the birth dropped to 50kg within about 2-3 weeks ... I breastfed and walked everywhere pushing the pram cause I didn't have a car ...

With the next 3 babies, I lost weight in the first 2 trimesters because of morning sickness but still suffered with polyhydramnious ... I think I put on about 10kgs each time by the time of their births (all 3 came early too) Now I have lost weight, but not really toned up IYKWIM? But I don't really get much of a chance to exercise, so ....

Like the others have said, don't stress about it You are growing a beautiful little human being!!