They are probably wearing a compression short or half-tight, which most running companies make and are very common for sprinters and hurdlers. These are tight lycra shorts but do not have the padding of bike shorts, so they're not heavy, in fact they're very light.
That said, people need to understand these tight shorts are of a heavier nylon material than the Underarmor type undergarment shorts meant to be worn under something. I have seen athletes wearing the undergarment shorts alone and that's not advisable and is banned by most high school and college meets. But Nike, adidas, Asics and most major brands make the running tight-shorts appropriate for wearing alone.