How to delete leftover files from aborted migrations?
You could use OmniDiskSweeper (free to download and use) to search for and remove big folders. To find the problematic area fast, run it in parallel on both your macs (to see the differences) and start at the top level.

If you don't want to install additional software, you can start by running du -sg /* on both Macs, drilling down into the directories with the biggest difference and repeating du until you found the problematic files.