I've purchased and worn these shirts for more than 15 years. My favorite Summer and travel/vacation attire for tops.

Sizes on the Columbia Bonehead or Tamiami shirts are intended to be oversized; with good reason. Increased air volume circulation is allowed by the outsizing of these shirts. Contribrutes to keeping your skin surface cool and dry which is especially important in humid climes; the fabric readily doesn't cling to you.

So while it is important to keep sizing in perspective, please keep in mind purpose and intent.
JIM IN SEATTLE