Quote Originally Posted by CavalryCmdr
Effectively you need to link invade and defend decisions, using the same triggers as the 'defend deep' when wanting peace, switch invade to 'buildup,' 'start,' or even 'none.' This should (mostly) solve the small army issue. Invade decisions will overide defend, so the AI is attempting to follow it's invade orders without an intermediate merge point (such as 'frontline') so the armies have no reason to merge until they reach thier goal. Changing the invade to one of these three leaves only the defend target point, so the AI's armies wil merege there instead.
Lol. I ran the test a few more times and was about to attempt to edit my post with a half-understood version of this :)

The above explanation is much clearer.