"Bigger muscles demand more blood," Paul Salter, RD, CSCS, and nutrition editor at Bodybuilding.com, says. "When you accrue significant muscle mass, your body adapts by expanding its blood flow network; thus, veins become more prominent in specific areas."
This means the more muscle mass you have, the more vascular you'll appear. Don't think your veins are big enough? Hit the weight room and build up that muscle mass, bro!
If you work out a lot, you probably have leaner mass
All the muscles in the world mean nothing if they're hidden underneath layers of fat. Sure, maybe you know they're there, but try explaining that to people who only see a muffin top where six-pack abs should be. Same goes for veins.