Well, I’m a guy, so I may have a different opinion.
But I do know that “M” or “L” can vary significantly from clothing vendor to clothing vendor.
Even the numbered sizes vary wildly.
I don’t know why the industry can’t see to get it’s act together on this - it costs consumers time and money and I can’t see how it results in any more sales than if they’d just tell you what you’re going to get accurately.
I agree with you about the men’s sizes, although women’s isn’t really much better. Impossible for me to buy my son or grandsons any shirts on line. Brand name means little any more. I’m one of those that needs to hold it up, look at it, feel the fabric, check the seams and stitching, determine possible shrinkage and try to picture how it’s going to look after it’s been through the washer and dryer.
It’s one of the reasons why I often just give up, buy fabric and make shirts for the guys myself. Unfortunately I’ve found that the quality of the fabric available seems to have also deteriorated over the past few years.