DWU2 actually the first step would be to shoot in Raw instead of jpeg. Jpeg is a compressed format. The size of that file is in your camera menu - typically basic, norm or fine. You are starting from a reduced state in all cases. After you process the image in LR you then export and again you elect your format. Quality equals size. Specifically answering your questions:

1. Is the size increase vs. the original in fact due to adding the edits before exporting? [[]] Yes, you started with a negative (jpeg, tiff or Raw) and process it.
2. What does "quality" even mean in this context. The 0% quality photos looked pretty fair on screen. [[]] Most posted images are only 72ppi
3. I've been using 80% quality for the exports I've done so far, considering it a good trade-off between file size and the ability to print in a fairly large size. What do you recommend? [[]] Print is a different world. 300ppi. You have to be careful to make sure to use "original" with nothing else selected. I have seen a lot of people crop the image perfectly and then have different settings in Export. Not good.

Hope this helps

There are a lot of things in a jpg file. Of course there's the data defining the jpg image, but there is also metadata including EXIF, IPTC, and comments. There are probably variations in metadata content depending on the jpg source.

I have seen a noticeable difference in the size of a jpg file depending on the software that generated the jpg. The following graph is included in my study on multiple writes of a jpg. I started with a tif, then saved that image to a jpg, then saved it again, and again........... The study (14 pages) can be seen at https://static.uglyhedgehog.com/upload/nt/2023/8/28/621559-compression_study_20220206.pdf



Note that ImageMagick changes its file handling above a quality number of 90.

Note also that in the middle range of quality there's about a factor of 3 between a file generated by ImageMagic and one generated by FastStone. If you really need the minimum file size I recommend ImageMagick (open source, a command-line program).

The image quality is, of course, subjective. I generally use 80 for my quality but it can depend on the image. A really busy image can show more artifacts at a larger quality number than a minimalist image (I have not yet studied that). Some images look just fine at quality 60.

Since you are talking about the 'original jpg' files, are we to assume that these are files direct from the camera? Or is some other software involved?