macOS Mojave has a slightly different behavior in its API that Yet Comic Reader rely on to get cache directory path. It introduced a bug that keeps increasing cache size, consuming free disk space.
Please update to version 1.6.4 from mac AppStore and remove the remained cache directory for retrieve your disk space.
The malformed cache directory is "/Users/<your user id>/Library/Caches/YetComicReaderCache/"
Normally Yet Comic Reader automatically manages its cache directory to take maximum 1 GB space.