AntiBigText prevents the text enlargement that usually happens on screens larger than 1596 pixels wide (i.e. those using the largest breakpoint).
When using AntiBigText, text sizes at the largest breakpoint should be identical to text sizes at the other breakpoints. If you notice a discrepancy, please leave a message on the talk page. See the breakpoints and typography Staff blog post for breakpoint details.
Add the following line to your global.css page (for personal use on all wikis). It must be at the very top of the page (with any other @import rules you may have) to work.
You can also copy the contents of the MediaWiki:AntiBigText.css page to your personal CSS page, if you prefer.