On this site (Photograph Works) the fonts display in the Admin panel more or less the same as they display in the front end.
But on another of my sites, (In The Apple Tree), the fonts in the back end are tiny. And the layout is all scrunched up.
I know that with some themes you can change the fonts using global styles. And with other themes you can use the Customiser. But those are methods to change the fonts in the front end. It doesn’t really explain why the fonts on my site look odd in the back end.
So I searched around, and this is what I found.
In the Admin panel click the three vertical dots at top right. A list will open up, and right at the bottom you will see ‘Preferences’. Click that and this panel will open up in the middle of your screen.
I have highlighted the section that reads ‘Make the editor look like your theme.’ In my case I unchecked it and now the fonts in the back end of In The Apple Tree are normal size and useable.
Photograph Works 