From what I've seen, the packs seem to be based around budget more so than content. So an expansion will be used to introduce a bigger budget addition, while a game pack will focus on an addition that doesn't require as large a budget, and so forth.
Take the upcoming horse expansion. Many people say the content would have fit in a game pack, and perhaps that's true, but horses are complicated and I'm sure require a substantial budget to do correctly. On top of that, they really do need a large world to get the most benefit from their added game play, so they ended up in an expansion.
