Are you just trying to find a way of getting another scan to confirm the sex?? lol Only joking.

How does she know it has a small head? Just from feeling your uterus?
From the outside my bellies with the 2 boys were identical. I was told the whole way through from feeling my uterus that DS1 was a giant baby, but DS2 was actually 500g heavier, and his head was was 5cm bigger. The outside isn't always a great indication. Having said that, if you are worried then another scan won't hurt, especially if it puts your mind at rest so you can 'enjoy' the rest of your pregnancy.