It's unusual. Typically MAME collections are distributed with deterministic compression and using automatic managing tools, so people can stay up to date withou

It's unusual. Typically MAME collections are distributed with deterministic compression and using automatic managing tools, so people can stay up to date without re-downloading large amounts of good data. 40GB torrents tend to hash 4MB chunks at a time, while a ROM might be only 8KB-- there's a high probability of a chunk becoming invalid. When someone updates to the next MAME version, they can download an "update" pack with only the ROMs that changed, and a tool will recompress them together with the old files so a torrent client can start seeding the new version immediately.