I know this is an 4 year old post but I have similar problem and googling takes me here.
I managed to locate the issue and in my case it was also the resolution that kept changing, I have 4k monitor on 200% scaling in windows and I usually play on 2560x1440 resolution with 120% in game UI scaling, but sometimes it launches the game on 3840x2160 resolution making the UI really small. But if I open the settings and hit accept like previously comment it makes the UI really big.
Just changing the resolution back to 2560x1440 it's becomes normal again, maybe it will help someone trying to solve this old bug.