In GTK2 the font size/type used for "system" objects (like menubars) can be overridden by parameter files - where and how depends on GTK level. There was a discussion on this forum about a year ago (and I added a tag #systemfontsize so you can easily find it) where I laid out those options and a user corrected the steps. It is not a perfect solution because the XTrackCAD code currently makes assumptions about font size when laying out the screen. This also doesn't affect the hotbar - you'll see my simplistic workaround there as well.
One major other issue is that on Macs and other screens with extra-fine pixel density (eg Retina displays), the OS and GTK often don't know about each others scaling efforts and that results in smaller font display than desire-able (it shrinks by 2x because there are 2x pixels) - that issue is hardware and OS level dependent. Changing the scaling is often an all or nothing proposition for all apps - you'd like the finer details for drawing but you can't read the text. The font is being rendered as though it was on a screen with half as many pixels per inch. There was also discussion of that previously, and I also tagged that thread similarly.
I'm not an expert on Windows system font settings and how they affect XTrackCAD, I'll punt to Martin on that.
One aim in moving to GTK3 would be to align more closely to correctness at least in the relationship between the app and GTK - by delegating the layout expressly to GTK making it all font size aware in all UI widgets. The issue between GTK and the OS and the OS and the screen hardware hopefully will improve over time - a lot of the Mac versions of that issue happened when when the Retina laptops first came in. I'm not sure what Catalina may have fixed.
