genetics and low bf. similarly i see guys that are big and fat but have large veins running down their bicep. however, i am very low bf, and even on dbols i have veins like crazy. drrman is saying that a lot of people choose certain aas because they think it will make you more vascular. its not the aas itself but the lowering of your bf while on it. var is good at this as you can add lean mass with calorie deficit thus dropping bf. i hear you bca though, i know a lot of dudes that never workout and have sick veins on their bi's and shit. on the other hand i was always low bf and the vein in my bicep never popped out till i ran gear. my shoulders have ropes all around and always did, but it took 10lbs of lean mass gain until they started to pop up in the bi area. vascularity is very strange to me.