You can probably set flags to download in lower bitrate formats. Some formats use a LOT less data than others, and usually the extra quality isn't really needed anyway if it's only being kept for reference. The difference between 1080p (or, say, 4K60) and 240p or 360p is massive.

Also the codec matters. AV1 is quite a bit smaller than VP9, which is in turn smaller than x264. Of course the downside is AV1 is harder to decode. For the purpose of well-encoded content, you can estimate that the bitrate doubles whenever you double both dimensions of the video, e.g. 720p would be twice the bitrate of 360p and half of 1440p.

FSM almighty, how much video are you watching to have that many favorite ones?

With the rise of cheap 4k cameras, it goes faster than you think. I download guitar training videos where a 25 minute tutorial can be 2-3gbs. I believe those are “only” 2k resolution. If you do not download them in a space conscious format or recompress (using mildly aggressive settings can drop them to 100mb), a handful of videos can quickly fill a hard drive.

Why wouldn't one download something like that in 720P video/196kbps audio?

I mean, there's a time and a place for 4K, but watching the zits on the face of a guy who tells you how to play C Am F G on a guitar isn't it.

Not to mention, most of those cheap 4K cameras won't have optics to utilize those pixels; no quality will be lost in 720p.

That depends, if the source is bright enough, those little pixels will have enough signal to work effectively.

My downloader defaults to the highest quality, so unless I remember to specify a setting, I get gigabytes of video for something that could easily be 480 quality. In the event I forget, I reencode with ffmpeg.

It's a reasonable default, but if one has gone to the trouble of setting up a NAS and scripts, might as well tick that box.

Why not archive seldom-accessed files onto BD-ROMs? They're cheap to store