When I click Sims 3 via Origin or from the game icon to start the launcher everything gets huge

Not entirely sure, but it sounds like the games are trying to run at a resolution that your monitor is not supporting, so they are changing the resolution of everything instead? Does it help if you adjust the games' resolutions from within their Game Options?

I've seen this phenomena a few times too on my desktop monitor, but only when the game stops responding and has to be force quitted via the task manager.

The native resolution of the screen is 1920x1200, but when this occurs it must be less than half of that. Everything is 2-3 times bigger than usual, and it looks so funny it always makes me laugh.

I have no idéa why it happens, or if there's anything you can do to prevent it, I just restart the computer and that brings everything back to normal again. :)