I won't disagree with either of your points, there are definitely other things that make for the snowball effect coming into play, however one of the biggest things that I have personnally noticed, and thus fixed temporarily via mods, is the recruit armies. It has made a massive difference in my games in the first years to have done this, the major countries that typically snowball for me (Vlandians and Khuzaits mostly) have been kept in check by the other countries and I very strongly believe it is due to the fact that the lords are able to actually train thier replacement troops rather than constantly rushing into battle with an army of recruits. Maybe this issue has been fixed in 1.3, I am just now starting a new game for the 1.3 branch so I can't say I have any real experience with it just yet but this has allowed those countries that are in multiple wars, and thus using every single recruit around (annoying if you are trying to find troops of the same type if I say so myself) to actually fight with decent troops rather than simply recruiting a full stack and then promptly losing the whole thing in the next major battle because it was mostly recruits.
It has been said that the snowballing effect is different for everyone so this may not be the fix for you but given that it has worked so well for me I would have to say that it is definitely a step in the right direction.
I have to say my biggest snowball offenders tend to happen within the first two or so years of the game, those that win the first early battles or get a lucky siege suddenly become top dogs by wiping the faction that lost that battle/battles and taking thier lords. Changing this so that those lords have actual troops in their armies means that even if they lose a battle they still have a snowballs chance in hell should they happen to run into another enemy lord later on, rather than simply having a peasant army get slaughtered by a trained army of troops that didn't see a defeat or Pyrrhic victory.