In article , PeterN
wrote:

I recently submitted two images to a competition. Bothe were 100 ppi and
measured, in pixels, 1020 x 768, both were saved at the same JPEG
compression level. Both files were saved as 8 bit JPEG.

there is no ppi in a jpeg file.

there is a tag that *suggests* an initial size, such as for a page
layout app (and that tag may not necessarily be used, depending on the
app), but other than that, the tag is meaningless.

ppi only matters when printing.

Yes. They intend to print certain images.

not very big if the source is 1024x768.

that's about 2x3" or so at 300 ppi, possibly 3x4" if they want to
accept a little lower quality (250 ppi).

anyone who requests a jpeg file at a specific ppi has no clue.

See above.

the tag doesn't matter. period.

the tag might be used to set an *initial* print size (or object size in
a layout app), but that's about it. that size can be changed at any
time, which will change the ppi, regardless of what the tag specifies.
most of the time, the tag is ignored.

if they're expecting to get an 8x10 out of it (which is what 100 ppi
would do), it's going to look like garbage because 100 ppi is *really*
low for printing. ideally it should be 250-300 ppi, depending on
intended viewing distance of the print.

the pixel dimensions are what matters, and clearly they're stuck in the
1990s if they want it at 1024x768.

That may be true, but that is not my issue. One of the images was 500k
and the other a tad over 1 mb.

it is your issue since you're participating in a competition run by
idiots who do not know what they're doing.

One of the images was a bit over 500 k. The other was over twice the size.

different compression levels and/or different amount of detail.

Originally they were both saved at the same level of compression. I had
to lower the compression level for the other to conform. Why, is my
question.

as i said, there are two factors. if compression is the same for both,
then the other factor is what will affect the size, that being detail.

in other words, one image has more detail than the other, whether it's
real detail or sharpening artifacts.