I studied and think I got a good grasp on the relationship between image resolution, DPI\LPI\PPI, and the difference between re-sampling or not. The only thing I can't understand is what I read (including on the Adobe website) about how in PS (and in general), you don't have the option to choose the file-size. This will be determined based on the other settings you chose (all the above, plus output format and other settings).
So my question is about 3-rd party software, specifically the Microsoft Power Tool "picture resizer" and I'll clarify...
I took an original image. File size is 16MB. Here are the specs:
I used the power tool to convert it to the same dimensions:
The result SEEMS to be the same quality, both to the eye AND in "image size" settings in PS (see comparison screenshots bellow) but the new file-size is now 2.42MB! (Both are jpg).
This is not possible in PS to export the same dimensions, with the same DPI, to the same format and to get an %84.88 decrease in file-size reduction.
So yes, there is definitely some algorithm in play here but obviously, the reduction in file size must mean that some file information was deleted, which I assume would be pixel information (compression), so how is it that the quality seems to stay the same? Is this just an illusion? If both files were now printed in the same size (30x80 cm), would they print in the same quality?
Comparison screenshots in PS: