I recently started using a Surface Book (This doesn't appear to be happening on my Win10 desktop). An application had text so small it was impossible to read, even when outputting to a large display:
I tried changing the font size in settings, and running
sfc /scannow to check for corrupted system files, but neither fixed the issue.
I also considered that it was just the one program, but after encountering this on another application, I realized - after some research - this is a problem with Windows 8 apps running on Windows 10, but only on specific displays or possibly display settings.
This is probably happening for all surface users, as well as other laptops. [Source]