Confused about album art size/resolution

Hi, I'm really struggling to understand album art as it pertains to what size(s) Apple Music actually uses.

In the past, I used to just click "Get Album Artwork," but that's not necessarily always reliable. (And doesn't that get art from the iTunes Store?)

I discovered this great site today, which scrubs the official Apple artwork from any album you want. He's got one for the iTunes Store as well. It seems like you can edit the URLs for these artworks to get just about any size you could dream of, and the high-definition ones are insanely large. But I'm not looking to massively increase my file sizes; I just want the generic, standard-size art that Apple tends to use.

Anyways, I have spent most of the day adding the 1000x1000 "standard size" (according to this guy at least) artwork to my albums. However, after embedding a ton of this artwork in my tracks, I decided to copy a few to the clipboard and open them in Preview... and was disturbed to find that they were all only 600x600. Not 1000x1000.

I also added 5 or so different tracks (from 5 or so different albums) from the Apple Music streaming service to my library; I checked these out in Preview as well. All but one were also 600x600, and the one exception was 600x595.

So, I'm very confused. Does the desktop Music app automatically shrink any and all artwork down to 600x600? If it does, how am I supposed to tell what resolution the original art actually is? I don't necessarily mind 600x600 versions (as long as they look decent enough), but I'm just seriously confused as to how this all works.

Aside from that guy's site, I also noticed that if you click Show Page Source for a web page of an Apple Music album, and select Images, you can see a 600x600 version and a 1000x1000 version. So his "standard version" of 1000x1000 seems to be the bigger of the two, at least when it comes to the actual Apple Music site.

I suppose I could delete all my art and just let Apple Music handle it automatically? But at this point, I'm not sure that's possible, without me deleting my whole library and re-uploading it. When I delete art that I have added, Apple Music seems to want to load that same art back into my tracks (sometimes it won't even do that); not sure if there's a way to force my tracks to use Apple Music's art...? Anyways, would greatly appreciate if anyone could help some of this make sense to me.