You make a good point, though I feel that the Psions are mostly responsible for that. I've always assumed that the Cabal devoted their entire existence to war, and they eventually encountered the Psions (maybe they originated from the same planet or were encountered on another, who knows). Seeing the Psions' intellectual prowess, the Cabal decided to enslave and eventually integrate the Psions into their society (most likely a caste system such as in the Spanish Colonial Empire) in order to acquire an intellectual and psychic power to complement their ground pounding form of war. Therefore, I believe that the Psions are mostly responsible for the Cabal's tactical and military prowess, though the Cabal admittedly receive credit for getting to that point in the first place.

I like your theory. Maybe the psions are part of the same race as the rest of the cabal, but have a stable mutation which gives them thier psychic powers at the cost of reduced size.

That's a very interesting theory, and it would certainly explain the integration of the Psions into even the forefront of the Cabal military machine (those Interceptors sure weren't made for Legionaries, after all). Either way, the combination of Cabal and Psion make for an extremely challenging foe (especially those Phalanxes with the no flinch modifier).

I know, I hate those no flinch phalanxs. Also the psions with the lightswitch (one hit kill melee) modifier are a nightmare

I absolutely despise those things. I'm convinced that the ultimate servant of the Darkness is not Aetheon, Crota, or any Fallen Archon, but a Psion Operative with lightswitch.