I have a 42 chest & ordered a large which fits great!  It's understandably a bit difficult to wriggle into but once I got the technique down it's pretty easy.
One nice surprise is how cool it keeps me while I'm cycling while at the same time pressing away those damn love handles that I'm self-conscious about.
I've yet to determine how long the (3) I bought will keep their fit but, for now, I'm very happy with the results.
A bit pricey but perhaps the materials are costly - just a guess & hope on my part.  Vanity tends to be pricey don't ya know :-)