I'm seeing the same thing with v 13.0.0.5. With UHD Creation running, it's shrinking MKV files on it's own regardless of whether you let it Fit to Disc or use the source size to create an ISO file, a BDMV file or are directly to disc even, if in each case there's adequate room on a 50GB blank Blu-ray disc. I've seen a 34GB MKV file shrunk down to under 9GB on several tries. I've gone back to version 12.0.5.1 for now.