Interesting. This probably has something to do with your OS reporting to Qt that the “device pixel ratio = 3”. Are you running a very high resolution screen and require HighDPI? On a HighDPI system with device pixel ratio >= 2, the Qt HighDPI support is actually supposed to make everything render bigger. The HighDPI support works for me on Windows 10 and Retina Macs running macOS. If I use an environment variable to disable HighDPI in the app, then the Shotcut log reports “device pixel ratio = 1” and things render small. So, it is strange that it is not working for you.
What happens if you export QT_AUTO_SCREEN_SCALE_FACTOR=0 and then run the shotcut launch script?
What happens if you export QT_SCALE_FACTOR=2 and then run the shotcut launch script?