TBH I have absolutely no idea why some babies are bigger... My sister and I were very different as children, though we have evened out a little now. She was a tiny baby, and continued to be tiny into her early 20's. I was a chunk, then a normal sized toddler, then a chunky school kid until around year 11.
My mum didn't do anything different, we just are who we are.
Like Tobily said, your baby is perfect And if she does turn out to be a larger baby (though remember that's not a given just because of what your OB said) you can still birth her beautifully without a problem.
All the best!