I have a small head compared to the rest of my body. Birth defect?

Range of normal. Head sizes come in a variety of sizes, primarily based on genetics. The head growth is also responsive to the growth of what happens inside. If the genes decide the brain & supportive structure will grow large, larger skull size will occur. If the genes can put it all is a smaller package, the skull will be smaller. Size does not determine quality in the package. I would consider it a nl variation.