A previous answer does not help me. I save a file after changing size (inches) and resolution from default of 240ppi to 300ppi, when I reopen the file reverts to the original size. I use a Mac 10.6.8 Photoshop CS5 extended 12.0.4 x64.
If all you change is the document DPI resolution setting all the changes is print resolution and print size. The document size remains un-changed you sill have the same pixels you have before the resize. Only when you resample(interpolate) the number of pixels you have for the images does the the file size change. In fact you have a completely new image not a single old pixel exist all pixels have been interpolated to increase or decrease the number of pixels the image has.