Why are my saved JPG file sizes larger than the original file size?

You are not resaving at the same compression level.

If you resave at a higher quality level, the file size will be much higher. I see you save at "maximum" here.

A jpeg is always heavily compressed, and the size on disk much smaller than the real, native size of the file. When you reopen a jpeg it is decompressed back to its full size, and so the compression starts again from the full size.

Jpeg compression is destructive and non-reversible. Ideally you should only save to jpeg once at the very end to keep the quality as high as possible. A much better workflow would be to scan to TIFF, do whatever you need to do, and then save out a final jpeg.

If file size is very important, quality 12 defeats the purpose of saving to jpeg. It's still compressed and (slightly) degraded, but you're not getting full payoff in reduced file size. If you go down to 9 or 10 you can't really see any difference, but you get a much smaller file.

EDIT cross post

Your example shows Maximum (12) Quality being selected. If the original scan was in JPEG format and that original was saved at a lower quality level than Maximum (such as 1 to 11), then the file size of the one you export now will probably be bigger. So it’s expected.

That doesn’t mean you increased the overall quality. All Maximum means is that more data is being used to describe the same image; it won’t magically improve it. To use an analogy, it’s as if the original fit in a 1 liter bottle, and now it’s being put in a 2 liter bottle. It takes up more space, but the content has not changed. (Or might be degraded slightly, since JPEG is a lossy compression format.)

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@dnemeth01 wrote:

I am having trouble archiving thousands of scanned pages due to the file sizes of formally smaller files.  This has been a problem for me for well over a decade.

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Keep in mind that Maximum (12) is not always a good choice. The quality difference between 10 and 12 is essentially impossible to see with the human eye, but the file size difference is vast. So a great deal of storage space can be saved, with no visible difference, by using Quality level 10 instead of 12.

If you’re choosing Maximum because you want no quality loss, JPEG at Quality 12 is the wrong format because, again, it is lossy. TIFF is lossless, but the files are much larger. So again the compromise is either JPEG at Quality 10, or maybe TIFF using ZIP compression.