i copy paste this from internet......
anyway, bulging veins are partially genetic and partially depend on
your muscle tone. if alot of blood is sent to your muscles, the
blood has to get back to the heart. as a result, the more blood
going to your muscles (from working out) the more blood ends up
going through your veins. girls can have veins too if they're
skinny and have good muscle tone.
anyway, not all guys have veiny arms, but ones that use their arms
in sports or gymnastics or weight lifting or something usually do.
also, veins are easier to see if there's little fat covering them
up.