The level of aggressiveness is quite random; each turn has one major wave of "barbarians" (the one mentioned at the "coming threat") and one or more secondary waves, which can be the same state as the first wave. When the latter happens, you are likely to see major conquests by a single power. If the secondary waves are more spread out, things can easily devolve into a status quo, so you're likely to see the other powers develop fairly differently from game to game.
The Parthians are pretty nasty though, as they have several waves of attacks, so will often end up in control of a large empire if they are not contained early on. Just be thankful the Carthaginians decided to go East instead of North.

I think the difficulty setting is about right once people have got a few games under their belt, but I should probably have made it adjustable. Something to put in when I get around to do a version 0.2.0, perhaps?