Font size changes when switching back from visual

Hi, I use font size 18px for the html editor. But when I switch back and forth from the visual editor, the font changes to a smaller size.

The font size button still says 18px, but the screen shows a smaller size.

I’m using last wordpress 4.5.2 and HESH 1.7.1

Any clue?

BTW your plugin is awesome.

Thanks!

https://wordpress.org/plugins/html-editor-syntax-highlighter/