I have been using the OnlyOffice with the Nextcloud UI, but recently, I decided to install the desktop client. I can’t really say for sure if the Windows and macOS version has the same issue; but in Linux, I get this problem. After installing the package, the default scaling seems to be set to 2. It is kind of annoying to do an extra step, but it seems like it is a one-time thing to do.
If you are reading this post and you have encountered the same scaling issue. Use the command below to launch OnlyOffice then close the application.
You would only run the command once and the OnlyOffice application seems to remember the settings. Hope you find this useful.