Don't forget ultrasound isn't 100% accurate. Also when baby is born it's head comes out usually with the chin tucked in and not square which uses most of it's head - it's designed to be born to make the most of the space. This is a snippet from myoptimal fetal positioning article on the main site:

"Position ideal for birth is when the baby is lined up so as to fit through your pelvis as easily as possible. To be in this position baby needs to be head down, facing your back, with his back on one side of the front of your tummy. In this position, the baby's head is easily 'flexed', ie his chin tucked onto his chest, so that the smallest part of his head will be applied to the cervix first. The diameter of his head which has to fit through the pelvis is approximately 9.5 cm, and the circumference approximately 27.5cm. This position is called "occiput anterior" (OA)."