Um, actually it does, unless you plan on shrinking the 300DPI version down to a tiny postage stamp size compared to the original 72DPI version?
Is that what you want to do?
No, actually it doesn't at all. Go ahead and try this for yourself. Export a RAW image with 3 different DPI/PPI sizes. The file size will be exactly the same. The only thing which affects file size is jpg quality and resolution size NOT pixel density.
If you'd like proof, or don't have the time for such an endeavor, here you go: Each of these files is the same resolution. I re-sized by dividing the original resolution by 3. One of them was exported @ 10 ppi the next at 150ppi and then finally the last at 300ppi. Each one has a file size of 705kb (well, 688.something according to Imageshack .. and imageshack does not resize) Just click on each link and look at the image properties.
Just to clarify, if what you're seeing when increasing the "perceived" pixel density is an increase in file size, it's because the software/program you're using is actually up sizing the resolution, and NOT the PPI/DPI. The PPI/DPI is really just a made up and non working element in the equation, which for all intents and purposes can absolutely be ignored up until the point of when you're going to PRINT.
http://img546.imageshack.us/img546/1200/10dpi.jpg
http://img847.imageshack.us/img847/3308/150dpi.jpg
http://img40.imageshack.us/img40/9079/300yk.jpg
And David, you're mixing concepts up a bit, and not reading carefully enough. You're making it out to be what you want it to be and not what it really is.
Furthermore,
What Preview is doing by what you perceive as increasing the pixel density for a larger preview (resolution) is not actually increasing the pixel density at all, even though it will assign the new file with an corresponding ppi value to which it can "
relate". What it is doing actually, is up-sizing the resolution all together. There's a very big difference between these two actions, as again... pixel density or dots per inch (as there is no such thing as an inch in regard to measurement here) ONLY relates to the amount of pixels needed in order to maintain a proper value for printing on PAPER.
I wish you guys would understand that, because it totally changes the perspective of this entire issue. If you knew this, then all you'd have to do is tell OS X to up-size the screen shot. Perhaps there is a way to do this via a terminal command.
Doug