Ah, clothing sizing. I think a lot has to do with manufacturers outsourcing to various foreign countries. There's no consistency in sizing or styles or much of anything.
I've been buying polo shirts--I looovvve polo shirts--from Lands' End for many, many years. A couple years ago i ordered some after not having done so for a while. I ordered SMALL. They were much smaller than usual. My weight does fluctuate a bit, but I didn't really think it was me. The next time I ordered MEDIUM. Would you believe that my new MEDIUMs were actually SMALLER than the old Lands' End polos that were marked SMALL? Some serious resizing had taken place. I do wish retailers would inform their customers, particularly when it's mail-oder.