How can I visualize deviation from the standard as a badge?

I have an icon-button on my website. When clicking on it, the font size can be increased/decreased.

I want to visualize that there is a deviation from the normal font size when the user clicks to change font size. I thought of using a badge for that.

However, I can't find a good way to indicate this change.

What should I display in the badge? I can only use text, also ascii symbols. Should it be a plus or minus, whether the font size is bigger or smaller than usual. Or just a "1".

What do you think?