For many racers the evolution from recreational cyclist to competitive bike racer takes time. Through that time you undergo many changes.
You lose weight.
You get a cyclist tan.
You shave your legs.
Your wrists and ankles get skinnier.
Your leg muscles get larger.
You have a patina of scars on your knees.
Then, at some point, you start to notice new blood veins, and that they get larger and larger. The longer you train, and the more endurance fitness you have, the more blood your body will produce. The more blood you have, the more access to oxygen your muscles have. More blood = more endurance.
When you have more blood, the body has to adapt to have the room to store all that blood. So, more and larger blood vessels and veins.
I first started paying attention to my veins about four years ago, fascinated by the changes. I have new veins on my thighs and calves. I used to just have one vein crossing over my right forearm, and now I have three. I like to think that the first one is what I needed for Cat 5, the second for Cat 4, and the third for Cat 3.
Also, after a long training weekend while doing another task, like cooking, I'm always happy to see full veins in my hands and arms. It tells me that all the training went well. On the other hand, if my veins are looking empty I know I may be a bit dehydrated and need to drink some water (unless it's after a meal, when blood goes to the stomach and gut).
Just another thing that I see totally different because of bike racing.