Most local clubs get their club jerseys from companies that seem to know their demographic. They fit snugly even over pot bellies, without being oversized elsewhere.
I haven't seen that kind of sizing in Pearl Izumi, which generally presumes "race fit" means what it says. Even at 5'11" and 150 lbs, I'm at least 5 lbs "overweight" in race fit terms, and it shows when I wear my size medium PI aero jerseys. They fit well, but my slight belly pudge definitely shows.
Check Garneau, which has some nice looking club fit jerseys that don't flap loosely in the breeze. I got one when I weighed 165 and it still fits well after losing some weight.
And AeroTech sizing caters to real cyclists, which is 95% of us.
With the cheap but good Chinese no-name/weird-name jerseys, it's all guesswork. I usually buy small or medium in those oddball shorts like Przewalski, Spotti, BBBppptttiii or whatever the heck names they come up with from falling face-first onto the keyboard. But with Black Bibs I get medium.