It's not so much gravity as some of you are saying. You'll really notice it when you start to work out.
When you work out your muscles naturally swell up a bit and harden (not flexed, they literally just get harder), this pushes the vessels out against your skin, in between the muscle and the skin, making them visible.
In places like your forearm, on top of your hand, and on top of your feet, it's just due to the absence of any fat or muscle coupled with increased blood pressure.