What makes a baby have a birth weight of 7lbs vs 10lbs?

What causes babies to have high birth weights vs. ‘normal’ or ‘smaller’ birth weights? I am trying to research this online but am finding more information about extremely high or extremely low birth weights vs. a more typical range of between 5–10 lbs. I will be talking to my doctor about this at some point but I like to be able to form educated questions and having more information helps me do this.

At one point a midwife told me that if you eat a lot of sugar and dairy in your last month your baby will be ‘bigger’. Now of course I know people for whom this was very true and others for whom it was not.

Any ideas?