The compression was already explained - I would add that if you’re not deep into scene groups, popular repackers are in general a safe choice, i.e. no malware included. Of course it’s never 100% safe, but in my opinion safer than picking from a list of torrents what could be the scene release.
I never had issues with fitgirl and Dodi, maybe other users can drop some more names that have proven to be reliable over a long time.