I've figured that you get a funny idea too. I went with a medium on my LLB binder, based on measuring but also by looking at guys on the net. Unless they give you their exact measurements-- and most do not. You really have no idea. They may be heavier in a different place than you.
I talked to someone and she (yeah it's a she, she is just has severe chest dysphoria) and she is a good 25 lbs more than me, yet she fits easily in a large, which barely fits me. Go figure.
--Jay Jay