My OB is like Mylitta's - never measures fundal height due to the enormous inaccuracies you can get. I had to have a scan at 33 weeks with DD1 and was told that she was enormous and yet she was born spot on average.

As for your weight gain, I think everyone is different. I have gone up and down a little throughout, but am still 1kg lighter now than I was when I fell pregnant. I've lost 2kgs in the last fortnight and that's taking into consideration that I am doing absolutely no exercise because I'm supposed to be on bed rest. So long as you're not consistently vomitting and unable to keep anything down, I think it can be just one of those things.