Sometimes the sizes of men's active wear can be confusing, and just because you wear one size in a Champion shirt doesn't mean that you'll wear that same size in an Adidas shirt.
The fit tip: If you're shopping in a store, try on the athletic wear. It's not hard, guys; women do it all the time. Alternatively, if you're doing your shopping online, read the measurements next to the sizes. Measurements and sizes match up differently depending on the brand. For example, an Under Armour men's running pant labels its XL as 36-39 inches at the waist, while Champion's Double Dry pants label XL as 40-42 inches. The moral of the story is that not all XLs are created equal. Of course you can return things, but who wants to make another trip to the store when you could be off to your next workout?