Early on, babies are babies and their size depends on measurements that are in the millimeters. There is very little variation at early ultrasounds because baby's size is a measure of its development and NOT its genetic capability for size. That kind of growth happens later on in pregnancy and while at the end of it all baby boys are on average 3.5 oz. bigger than baby girls, this is really not something that is visible that early in pregnancy.

It's not easy to measure a fetus that precisely and so it's relatively common for there to be a day or two variation just because the techs goof it up a little.

Now, just to be sure this is understood, just because something is an average difference does not then mean that "all boys are bigger than all girls" this is an average, just like men on average are taller than women, but there are still plenty of women that are taller than plenty of men! My daughter was my second largest baby, bigger than 3 of her older brothers!