Gruber broke this concept down in a November '05 post titled Full Metal Jacket (under the Display heading), but to summarize: most of the dimensions of elements in Mac OS X (and other OSes to my knowledge) are defined in pixels - the menu bar is 22 px high, for example. This explains why things 'seem to look a little smaller' when you move from the 1024 x 768 dimensions of a 12" display to the 1440 x 900 resolution of the latest 15" PowerBook G4 or MacBook Pro displays. Conversely, if you decrease the resolution on the machine you're working on now, things will look a bit bigger; you have smaller resolution and fewer ppi (or dpi) on screen, so some elements change size. This can become a problem in the context of notebook displays and their resolutions - if you take the 15" MacBook Pro's resolution higher than 1440 x 900, things could become smaller than what many might consider usable (these same rules apply to Windows and I believe Linux as well). Further, you can't just keep increasing notebook display sizes like you can with desktop displays; I've heard of the 19" notebooks Engadget has come across, and I personally don't consider a 16 lb computer worthy of the 'portable' adjective.