When it comes to the head-size measurement, your pediatrician is simply making sure that your child's brain is growing at a healthy rate. A head that doesn't grow fast enough can reflect poor brain growth, while a head that grows too quickly can indicate possible developmental problems.
Again, genetics play a role, so what's "normal" for your baby depends on what's normal for your family. In our family, we've always had trouble finding hats big enough to fit our "generous" heads—and sure enough, my 18-month-old daughter wears a size 4T hat. Between ages two and three, you can expect your pediatrician to stop measuring your child's head circumference, because most of the brain's physical growth is complete by that age.