It's different for games...the way I understand it (which might very well be wrong), polygons can pretty much be displayed at an arbitrary resolution. The things that get affected by the higher definition are the 2D elements like textures and FMVs...they need to be in a higher resolution to avoid appearing pixelated/fuzzy, and do take up considerably more space. You'll notice that FMV-intensive games such as Blue Dragon and Lost Odyssey come on several dual-layer DVDs, whereas a game like Mass Effect which uses primarily realtime cutscenes fits on a single disc.
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