When I began authoring Keynote presentations in the early 2000's, I went with the slide resolution/size of 800x600 pixels. VGA was 640x480, and who would ever need more resolution than could be shown on a TV screen? I figured I was future-proofing by dialing the resolution up a notch to SVGA, knowing full well that the resulting documents would have larger file sizes.
As fortune would have it, things change. I noticed that the interactive QuickTime files I created from my Keynote presos looked a little chunky when projected digitally. Keynote uprezzes presos to match the projector's capabilities pretty well, but QuickTime isn't as clever. (Neither was I for not knowing that.)
Last year, I began transforming my 800x600 presos to 1680x1050 (WXSGA+). The new aspect is a better match to the MacBook Pro's own display (no black bars). Keynote does its best to scale everything up. But with animations and grouped objects, it doesn't always work out. So tweaking has to be done. Sometimes, the tweaking is nontrivial.
I launched a raft of updates into the momentum unit. And into the UCM/gravity unit before that, and into the Newton's laws and motion units, too. I tag the links with an "HD" when I remember to. If you've been using the old presos, feel free to grab the new versions. The files are bigger, but the added resolution is worth the wait!