Guranteed* to improve run performance by 3-5%. They are quite fitted, but not tight. Debs and Kaz describe the size as "a stylishly snug fit, but wouldn't wear it after a big lunch". If you like more room, then size up.
Debs wears XS. Kazy wears S. Disclaimer: Neither are blessed in the boobage dept.
*Just kidding! But you'll look sh*t hot on the trails.
