My nipples are very small for a male why is that?

Male nipple size. Nipple size varies enormously in both men and women. In teens there are all kinds of hormone changes taking place which determine the appearance of breasts. Cool temperatures can cause nipples to shrink & harden as can sexual arousal or physical stimulation such as from tight tank tops or t's.
Small nipple/man. There is no medical reason for it.Thats the way you are and size of nipple varies even in women who have a reason for nipples as for breast feeding, but in men it does not matter at all.You have to live with it.