Since you have this ever since you can remember and nothing happened, then there is no reason to worry. Normally the head too increases in size, but if it does not you don't necessarily have some medical condition. But the general principle always applies: when in doubt, ask a doctor.
Problem may be serious, it sounds like early stage of Erectile Dysfunction. It really depends on your age. During erection blood flow enter in penis which makes it bigger, if there is not enough blood, it remains the same size. Another possible reason is problem with muscle contraction of the part of penis which remain inside the body.