Hi sj
i measured big all the way through my first pregnancy with my son. i was consistently 2/3 weeks ahead at every measurment. They sent me for a scan at 35 weeks, but the scan showed that my measurments were correct for dates and that the baby didnt look massive so it was quite possibly just the way my bump had formed. As it turned out i had my son early just 6 days later at 35+6 weighing 5lb 13oz. Chances are if he had gone full term he would have been a good 9lber but thats not excessive enough for me to measure over.
In comparison, my 2nd pg i measured correct right through and went to 39 weeks giving birth to another boy weighing just 7lb 2oz.
I wiouldnt worry too much, it could be something as simple as how your bump has formed, or even where you placenta is sitting...if its at the front this may add to bump size. Dont worry, your scan will show up if your dates look wrong or baby is going to be a big one!!
Good luck
caroline x