/a/ mostly downloads off of Nyaa Torrents, at
Nyaa.se, which is a torrent tracker dedicated to anime and other Japanese media. It's also run by fansubbers, and a couple of their mods also post on /a/.
Alternatively you can grab the same releases off of IRC by going to #news on Rizon. Google "anime packlists" and "how to XDCC" if you want to use IRC.
For quality, file size is not really a good indication. You have to look at the encoding information provided, like whether its 10bit or 8bit, Flac or AAC, TV source vs BD source vs stream source.
Also some general information:
Horriblesubs is not a real subgroup, it's a bot that rips various simulcast streams (i.e. Crunchyroll, Funimation, Daisuki, Hulu etc). It rips the video with the subs intact and uploads them as an .mkv to Nyaa and IRC. Horriblesubs doesn't really control the quality or size of the files they upload, their process is almost fully automated. Because of that there's little point in archiving HS releases.
Horriblesubs is fine if you're watching a show weekly and don't want to wait for slower fansubs on a TV raw (not that it's even an option for most shows anymore), but if you're watching a completed show that has BDs available, you should always take those instead. Even if you plan on deleting the show after you finish it.
If you look around on Nyaa and see stuff tagged Deadfish with smaller file sizes and mp4 files, it's because they just reencode the fastest release (usually Horriblesubs) into 8bit and mp4 so it can run on toaster computers.