[--- UPDATED: 9/3/2007 12:35:31 PM ---]
This is probably because of a bug (or "feature", depending on your point of view) of your video driver. You have a high-end graphics card, right?
Solution: go into your graphic card settings and set it to display at 96 DPI (or, if that doensn't work, try another DPI setting).
Sorry, we don't know where in your driver software you need to look - this varies from video card manufacturer to manufacturer.
Basically, what's going on, as far as we can tell, is that your video card driver has been programmed to be "fast" - so fast, that it's not stopping to realize that the PC is running a standard windows program, not some game program where other resolutions may be advantageous. Hence, it's trying to "optimize" SimPlates like it would a game, and doing a poor job of it.