Baby's measurements

I'm 28w2d, had an ultrasound they said she weighs about 2.4lbs. I have a doc visit today and they measured my belly of course and they say it's at 34. My question is, with me being a plus size woman how do they know it's the baby and not me? I haven't gained any weight this pregnancy have actually lost but just curious as to how that works out because I thought you were suppose to measure about the same as the weeks you are.