I've struggled for years with loose fit shorts on long runs when it's really hot and humid. It seemed that no matter what I did, I always ended up with chafing. In an attempt to thwart the dreaded chafing, I've tried many fitted shorts, but just couldn't find any that would actually stay down. I am so pleased to have found the Little Black Shorts! They have a good amount of compression AND stay down. I've worn them on numerous runs (with washing and air drying in between) and have only good things to say about them. I would love to see them in more colors!
From a sizing perspective, I recommend ordering up if you're in between sizes. I'm 5'6", 145/150 lbs and am very pleased with the size Large (I've already ordered a second pair!)