What causes blue veins to show under your skin on your thighs and chest which started since weight training?

Result of Wt loss!: It is due to loss of fat under the skin, and as the underlying muscles get bigger with training, the veins become prominent. The veins are located just under the skin, then fascia, under which the muscles are located. I am sure yeins become prominent elsewhere else also, but one sees them easily on the thighs and chestwall, while those on the back or back of thighs/legs are not seen easily.