A user complained that on mobile, he sees a certain font size while he's typing, and then after submitting his post, the font size is smaller. Is it possible for the font size to stay the same after submitting the post?  

This is by design. Font size is reduced on mobile by the responsive design of the software

Thank you. So there's no way around it? Can I not increase the font size of posts on mobile devices, so that it matches the font size that the user is seeing in the text editor while typing? 

The only way in which to change that would be by amending your theme

1 minute ago, Marc Stridgen said:

The only way in which to change that would be by amending your theme

Ok thanks.