I put on 4kgs last pregnancy.... DS weighed over 4kgs. LOL
I am bigger though, and my appetite went down, so I wasn't eating and I was sick everyday until after giving birth. I don't think it's a huge issue, as long as everything else is going well, that you are eating well, and that you feel fine.
You should consider yourself Lucky... I have heard of some people who gain a Tonne during pregnancy.