Everybody is different, some people can gain a lot in fluid and some people don't.

baby #1...I put on 7kg
baby#2...I put on 7kg
baby#3...I put on 8kg
baby#4....I put on 7kg

A friend of mine is a tiny size 6/8 and puts on over 20kg during pregnancy, but goes back to her little size 6/8 not too long after.