Arsthbt

My question is simple:

Why do most radio buttons not fill their entire outer circle?

Example of standard radio button:

(Unusual) Example of an entirely filled radio button:

Is this for some skeuomorphic reasoning or something entirely different?

Bonus Question:
Is it ever acceptable in an interface or design system to use fully filled radio buttons?

Radio buttons were inspired by the physical radio buttons ( obviously enough ). Buttons were popping out from a frame or were "3d". I would consider these as signifiers. Those signifiers made their way to the first interfaces. Practically a button popping out from a frame that when pressed would cancel all the other pressed buttons.

As of today, the radio button should also be recognizable even after it is checked. So having it according to the "standards" would be best. Even though I have seen examples of radio buttons like your example and inside the context I did identify them as radio buttons for some users it might cause some confusion or it might not but some user testing might be needed.

This is a nice article about the history of radio buttons: https://www.jitbit.com/alexblog/242-the-history-of-a-radio-button/

As said in the article:

Radio buttons are named after the actual buttons used on old radios to switch between radio waves and, sometimes, preset frequencies. When a button was pressed, all other buttons would pop out.

So, it was a skeuomorphic reason.

I believe a fully filled radio button would confuse users, based on the principle of "Match Between the System and the Real World". It states that:

Interfaces that follow real-world conventions and make information appear in a natural and logical order demonstrate empathy and acknowledgement for users.

Users are used to radio buttons in this format, so unless you have a really good reason to change that, I think you should use the standard.

I guess a completely full radio button can be easily confused with a bulleted text:

Whereas an empty circle perceptually gives the feeling of incompleteness: needs filling or has to be filled:

Even in many cases one of these rings is already filled, which increases the feeling that they must be filled (checked):