I put on at least 12kgs in my First Tri, Jules...
I think my body went a bit crazy to finally be having good (and lots of) food. Not to mention I was at least 5kgs underweight before I fell PG. I believe my massive gain was definately due to a stuffed up metabolism...
I think they say a gain of 10-15kg through your whole PG is about average (however I put on 3 times that much!)
Don't worry about it though as everyones bodies work differently, and its just great that you are eating properly, and bubs is getting proper nutrition! Your body might just want to hang onto a bit of extra fat just so it has reserves... .