Wow, I was a little worried since these did not have many reviews but I'm so glad I bought them! They're made from thick, sweatpants like material and the waistband is also very thick and sturdy. I'm 5'5 and 130 pounds (59kg) and got the Large, which ended up being perfect. I'm normally a size 4/6 or 28. I wouldn't recommend the Large to anyone above a 28 in. waist though, 29 in. would probably fit but you might be pushing it. My assumption based off of how it fits on me is:
L: 26-28 in.
M: 24-26 in.
S: 24 in. and under
As for the legs of the shorts, they're loose and give a decent amount of room. My legs are chubby but proportionate to the rest of my body, so do with that what you will. If you have thick things you might want to consider sizing up.