... do they have a bald area underneath?
Our cat has a narrow bald ring that is visible when you stroke her fur backwards. I'm pretty sure the collar isn't too tight (I can easily get fingers underneath, and if it was any looser she would get her paw caught when scratching). The vet has never mentioned any concerns at our annual check. But my teen son googled it and got himself worried, so I thought I'd check for experiences here.