Hello,
Thank you for reporting the issue with the large output file size in Upscale mode.
Regarding the situation you described, please first download and install UniFab version 4.1.8.0, then try upscaling the 1080p video to 4K again and check whether the output file size is still unusually large.
If the same issue persists in version 4.1.8.0, please provide screenshots of the output settings, the duration and size of the source file, the size of the output file, and the relevant log files so that our technical team can investigate further.
Thank you for your cooperation.