Why are guys veins so large and bulgy compared to women's?

More exercise! Strenous muscular work demands more arterial blood flow, and therefore more venous return! I have large veins on arms (wresting, weighltifting), but when i took up mountain biking, the veins hypertrophied on the legs!
Distribution. Women have an extra layer of fat under their skin which makes the skin more supple and smooth. It also is a matrix in which the veins are distributed and so women's veins are far less prominent. The exception is the type of diet that female body builders undergo causing the skin fat layer to thin out and making the veins more prominent in this group.