I still find it difficult to understand this bit really - "The dpi setting is just one little number tagged in the file somewhere."
See if this helps.
Imagine that you have a hundred tiny ball-bearings (representing pixels). You can lay them out on the carpet in a tight ten by ten ball-bearing grid so that the square they form is only an inch across (high resolution) or you can spread them out so that they form a ten by ten ball-bearing grid which is a metre across (low resolution). Despite the two squares being different sizes, they are each formed from the same number of ball-bearings which will of course take the same amount of space to store away (file size).
Imagine now that when you do store them away you add a little note specifying how closely the ball-bearings should be placed when they are laid out again (resolution). The note can specify a low resolution or a high resolution but it won't take up less or more space itself because its just an instruction. That's what the other Joe means.
after all these years, I'm still learning nifty little tricks.
Me too! Here are a few handy Photoshop viewing/zooming tips in case you weren't aware of any of them.
Double-click the zoom tool icon in the toolbar to zoom to 'actual size' - ie each pixel on screen represents one pixel in your image.
Double-click the hand tool icon in the toolbar to zoom all the way out again.
Drag a rectangle using the zoom tool to quickly zoom in on a specific area.
Hold down spacebar to temporarily switch to the hand tool - very useful for scrolling around while zoomed in.
Press 'tab' to hide/show all active palettes and the toolbar.
Better stop there. Hope you find something useful.