I first discovered this bug in Kubuntu 17.04 using Plasma 10.5, then checked on the latest KDE Neon dev unstable build to check if I can recreate it. Some fonts, like Garuda and Ubuntu (I didn't test any other font) make the checkboxes in the network menu in the system tray bigger than they should be. I've uploaded files to Imgur to visually help with the bug. https://imgur.com/a/RooLk
This may be fixed by: https://git.reviewboard.kde.org/r/125773/
That bug has been discarded. I don't see a fix. I continued my testing and things got worse after logging out and back in. You can see more screenshots in this folder I've created. https://drive.google.com/folderview?id=0B8Sna2alGiTLUHBsRjg5RG5WbG8
As you can see in the screenshots some things look normal while others are blown up depending on the font I use. Only the default fonts work as intended, Ubuntu and Garuda both have issues. All apps looked normal, only some plasma shell elements are blown up.
The review was discarded, but the people commenting didn't really sound like they fully understood the patch. My note was for Marco and other devs, not directed for you. and yes, it's just plasma components that try and do a thing with font size.
The layout bug with the checkboxes not being centered has since been fixed. As a result, I'm not sure there's a bug left to fix here. Yes, the checkboxes are a bit bigger than normal, but it's actually somewhat helpful in this particular UI since it increases the size of the click targets