I came across this problem after updates to Photoshop 21.0.3 and 21.1.
In Preferences, Interface, Presentation, UI Font Size had been reset to default.
I noticed this straight away after the last two updates, because when opening Photoshop after updating, the Font Size was Tiny and it hit my eyes straight away.
Once I've reset the Font Size to my liking, there's no problem with the font size again when opening Photoshop.
As mentioned, this only ever happened after the last two updates. The first time, well I thought it was a glitch but having it happen a second time, makes me wonder. I did mention it in passing after the last update here:
https://feedback.photoshop.com/photoshop_family/topics/photoshop-21-0-3-is-now-available
Curious really, as the update only affected the UI Font Size and no other preferences.
I have the latest Windows OS and latest drivers.
Cheers
Rosa