Have you got any idea what accounts for the difference?
In a word: no. I can make a few guesses though. For instance, web sites that support providing optimal info to their visitors might opt for more obvious/larger notifications, while those that don’t might want to stick to the lower size limits of the requirement. Alternatively, it could be ‘just’ a web designer’s choice, or simply a software default. Of course, there may be other reasons too.
Apart from this, it’s good to know that
the GDPR notification requirement is separate from the actual setting of cookies,
regardless of what’s suggested about that in the notification. You can readily see that by having Safari’s Privacy/Manage Website data pref tab open (or the equivalent function in other browsers, if available): from what I’ve observed, cookies get set independently from the user’s response to the notification. It doesn’t matter whether you agree with or just dismiss the notification, your cookie is often set before that choice is even made.