They're designed it seems for a man who's 5'10 and 170lbs or something and then just scaled linearly in both X and Y. That's not how human beings scale, though.

As a guy who's 6'3 and a bit more torso than legs I feel like I would need to be at least 250-275lbs for an XL shirt (which I need to wear so it's long enough) to not look like a sail.

This isn't a problem unique to women, it's unique to anyone who's not a 5'10 170lbs man; the mannequin they designed the shirt for.