I don't know if this changes much at higher difficulty levels as I typically dont play above normal as I get the most enjoyment at that level... but why don't minor races make more of an effort at expanding?
They almost always just tend to work at defending thier world but not really expanding.. indeed the only Minor Race I have ever seen get more than one world was the I-League when they appear. It would be really refreshing to see some of the Major races get beaten down and taken over by a minor race and have the minor race become one of the major players in a game.
I've even tried giving them worlds near thier own space in an attempt to encourage them to expand and they pretty much always without fail end up abandoning the colonies within a few turns..which is just plain odd.