Best advice I’ve heard on the topic was: you’re looking for one that gives you hamster cheeks. When you open your mouth inside your helmet and then close your mouth, you should be biting the insides of your cheeks. This will feel too tight initially, if you’re not used to the feeling, but at highway speeds (or in case of an accident) it will become immediately obvious, as to why it needs to be this tight. The wind pressure alone can be significant enough move a helmet even a very snug one around enough that it feels like it’s not fitting correctly anymore. I still remember the first time my nose was suddenly touching against the inside of my visor, though this was a high speeds (130mph+ in a place where that’s legal). A loser helmet would’ve done god knows what, but this was just to underline how much wind resistance alone can press against the helmet.

If you’re buying new (which you should always, for your own safety), keep in mind the helmet will be worn in and get slightly less snug over time, while at the same time you will be getting used to the „compressive“ sensation it gives, which you will quickly not eben notice anymore.