There are alot of factors that play into it. Skin thickness, which is based on genetics, is a HUGE factor. Different compounds also cause a higher increase of red blood cells, which thus expand the veins.
In my case, I'm a skinny ass ectomorph all the way. When I dont eat I look like I went thru the holocaust. The only kind of weight I put on is muscle, never fat, so gaining fat isn't a concern. I've got paper thin skin as well, as do many ectos. I also bulk year-round.
I'm a test & trenbolone kinda man myself. Causes a huge increase in RBC. Test for the base and mass building. trenbolone for the vascularity, tremendous strength, and grainy look. Sometimes kickstarted with Turanabol.
In fact...i'll be on in just a few weeks. Watch out bitches