I'll start by saying that at 27, these are for me. Why? Because for the money they are all around the better buy in my opinion. This review may not target the audience the briefs are designed for, but it will give you an idea of how they can fit just about anyone. And being its the first review I'll give as much detail as possible in hopes of it helping.
I wear them in XL, which is a 32-34" waist according to the package. However, these are 40% polyester and polyester is quite a bit softer/thinner than regular cotton, so they have more stretch to them. I'm 5'9" and weigh about 180 lbs and they fit perfectly. They're a little snug (barely, almost like a pair of underwear straight from the dryer) for the first half hour or so that they're on, but they aren't tight at all. After that half hour they retain their shape and you dont even know you have them on. They fit like the mens classic low rise brief but without the overly wide crotch part that drives me crazy. They're much cooler being they are thinner and 40% polyester, the support for the package area is great and NOT constricting, everything is just held in place well, and they are sized large enough to fit more than just boys. The only downfall is the waistband. Its not soft by any means. It doesn't dig, but its just a rough elastic material that could be imrproved upon, or covered with a material.
So to give another example on fit with these being they arent 100% cotton. I also own (and wear on occasion) the boys classic brief (900278). When I was up around 190 lbs I could wear them in XL. They were tight, but would still work after a little moving around. Now at 180 they are a little looser, but not like these ones. I'm quite confident I could wear the performance briefs up to 200 lbs with a 36" waist. They would be tight, but they are about 20% stretchier than the classic versions. So yeah, they fit generously because of the sizing. They aren't oversized and too large, they just provide a lot of stretch and are very soft. Worth every penny compared to the mens version in my opinion!!
I bought these in a size Large for my 9 yr old, 107 lb son and they're not working out for us. The fabric is soft and thin…perfect for a boy who runs hot, but it's just not the right fit. My 12 yr old will wear them, so I won't return them, but I had hopes of success when the (very nice) customer service lady said that this is what she buys for her chunky 9 yr old son too. Now I'm back searching for 'husky' underwear.