it should give you a clearer idea about the difference between PPI and DPI--because it can be confusing.
see the attached files and note the difference of each file size in megabytes.
your math is correct--but 'resolution' affects the overall file size AND that is what you should be concerned about.
It is much easier and more precise to resize jpeg files with Photoshop or another raster image manipulation program.
I'm working on a project that requires 300dpi JPGs at size 2.75" x 3.75".
When I look at the document size in document setup, it shows up as 2.75" x 3.75". So far, so good.
But when I try to export, the final image is 975 pixels by 1276 pixels. Which by my calculations ain't right. (Should wind up at 825 x 1125, no?)
When I change the units to pixels in the Document Setup window by the way it reads at 198 x 270.
This corresponds to 72 PPI, which is 2.75" by 3.75" expressed in pixels/points; this just expresses the actual size regardless of resolution.
Are you creating vector artwork to export as JPEG at 300 PPI? If so, just set the resolution as part of exporting.
Or are you working with a raster image (JPEG?) placed in Illy? If so, you cannot just change the resolution and expect the same quality.