Hi Grabbie,
I actually was a little underweight when I fell pregnant (58kg), - probably due to an unhealthy lifestyle. However by my 9th month my weight had escalated to 97kg! That is almost a 40kg gain...
My sister who was pregnant the same time as me was around 105kg when she became pregnant, and only put on another 9kgs throughout her whole pregnancy, without doing any extra excercise etc. She did have a few problems with high BP towards the end of the pregnancy, but that was really the only complication. She also had an easy labour (her 3rd time) even though the Dr's told her to expect it to be harder than her pervious ones, due to her weight.
I think maybe you are at less risk of putting on lots more weight in your prenancy if you already are overweight. Whereas in my case my body was previously a bit starved, so when I started eating heaps for my baby it would store every little bit of extra fat - I guess it wanted to store it as reserves just incase I stopped eating again.
I still have 20kg to lose from my pregnancy weight, and it is very hard going, I have been stuck at the same weight for 2 months, even though I am eating healthy and excercising more.
I think the only reason I gained so much is because I quit smoking. I also had to eat every hour to stop myself from throwing up, and I also overate heaps too because I felt that being pregnant was an excuse to eat as much as I wanted.
I found the one thing that stopped me gaining weight so fast was when I joined the Pregnastics class at the RBH. It is low impact aerobics (at your own pace), as well as pilates, excerball and stretching, - the class instructors were physiotherapists. I did classes twice a week from weeks 23 to 37 of my pregnancy, and if I hadn't I probably would have put on another 10kgs! Maybe they hold classes like that somewhere near where you live? It is also great to talk to other pregnant ladies there.
Swimming is also really good - its so much easier to excercise as the water makes you so much lighter!!
So anyway, even if you feel like you're always hungry, you should listen to your body and your baby's needs, dont worry too much about gaining extra weight, as good nutrition for bub is so important.