Only dragging map thumbnails onto map?

I have an issue persisting across multiple games I've launched, where any maps I try to bring in from the art library are as small as thumbnails. Even grabbing them and holding alt, dropping them on the art layer, etc, nothing helps. I don't think this has been an issue for me for long, I believe that last October I was buying map packs off the marketplace and making some wonderful sets with them no problem, so it must be a new issue. Also images brought in from my desktop or even the recent downloads bar on Chrome (which is a really nice touch!) come in full size. So I'm not sure what's wrong.

This sounds like normal behavior.  The VTT doesn't respond to the size of an image.  It will come in as 1 grid unit on the token layer, or 3x3 grid units on the map layer.  The only way you should get a full size image is if they were part of an addon pack where they were already set up as pages or were linked as a token to a character sheet in the journal, since the token would then retain the size.

Conversely, many images will come in at native size when first dragged in from outside Roll20 (say from your desktop). This might be the behavior you are remembering that was "different before".

I guess you're both right. It's been a while since I've GM'd on here, and I think it was back in summer of '19 that I started a new campaign focusing on a single homemade battlemap, and later one from the shop. Thanks for clarifying, I was really afraid I had gone crazy. Good rolls to you!