Gestational diabetes can cause your baby to put on weight faster so this may be the reason, but as the others have said its not always an accurate guide, they reckoned i was having a monster baby but he was 8lb!
Same thing with me...i was told my son was gonna be at least 10lbs and my son was only 6lbs 15oz
Fundal height measurements are not accurate. I was told I was measuring ahead with my first son and that he was going to be over 9 lbs. However when he was born he was only 7lbs11oz. I wouldn't worry