You have manually set the typeface to be big in the sidebar. Click on the ‘x’ to reset it to the default, then add custom class and adjust the way you want.
OR, just adjust the size in the sidebar. When you adjust it in the sidebar WHILE in mobile mode, it will only change for mobile view.
Welcome to responsive web design. Using the sidebar is the easiest way to adjust text sizes on all three device size choices.
I find using classes will give you a lot more control over everything. I use it a lot to control padding and margins between the different device choices. Once you have a class you can easily reapply it throughout the site.