firstly, scans are BAD at predicting babies' weights, especially bigger ones - mine was supposed to be a whopper, but he was a nice, average-ish 8lb 2oz. they are really better at picking up when a baby is not growing at the right rate.
secondly, she might be right - the extra weight pressing on the cervix could help it to dilate more efficiently, but there are so many other factors affecting the birth it's hard to say for sure. i can't comment on how easy or otherwise it is to give birth to any size baby, as i had a c-section (nothing to do with his size - i had too much water and he was in the wrong position).
thirdly, hope all goes well for you x