I just posted about this. DD was 10.5, and I just had a sizing U/S with this LO, and he's 8 pounds now, estimating about 9.5 to 10 at full term.
It's also a genetics thing with me -- I don't have GD.
I have also only gained 11 pounds -- more than your 1, but I was definitely wondering before today how I could be measuring 3 weeks ahead and only have gained 11 pounds. Now I know LO is 8 of that...so I guess once he's out, I will find I have lost weight underneath the pregnancy! Probably same with you.