It is very common to need to customize the styles for font resizing. Depending on what types of units you use for font sizes in your theme and how diverse they are, you’ll have widely varying results from the default styling shipped in the plug-in.
You can create your own custom style sheet and pass that to the plug-in; here’s an example of how to do it: https://github.com/joedolson/plugin-extensions/blob/master/wp-accessibility/custom-fontsize-css.php
The font sizes themselves would need to be customized based on your actual theme environment.
