Well, people don't seem to be taking some stuff into account, namely.

Orks don't run out of bullets. They have a group unconscious. If they need bullets for shootey, more and more orks start making bullets instead of doing other orky things (raising squigs, fighting, fighting, repairing/making vehicles) etc. It's not just mekboys, mekboys are just necessary for the larger/largest vehicles like gargants (and then, you only need like one mekboy, it's just you need a grand mekboy or 4 out of 5 gargants explode when first powered up - orks don't care, though).

The Zerg are used to fighting sane opponents. One of the biggest advantages of the zerg is their coordination and control, they don't fight unless they can human wave the enemy to death, and they are not used to fighting something as well or better coordinated them. Ork battle strategies are not strategy as we know it, they have no leaders, poor communication, poor everything, but they always show up in the right place at exactly the right time. That is because they are psychic, and have a group unconscious that guides them to the right place in nearly any battle, and they are too dumb to ignore what their unconscious urges send them towards.

Orks don't just get bigger when they fight, they make more orks. Ork reproduction is directly linked to conflict. Also, ork spores survive tyranid biomass extraction (although they can't grow without biomass) and imperium virus bombing - i'm guessing zerg creep wouldn't faze them unduly. There is a reason when faced with orks every foe of the orks works to wipe them out as fast as possible - when you don't, you end up with more orks than when you started with, and they're bigger, and their tech works better (not only do the mekboyz build bigger stronger stuff to fight bigger stronger opponents, their psychic powers (that power their tech) amp up from conflict). The Zerg tend towards slow destruction of enemies on the ground, and to prefer to build up than to throw everything into an attack.

The less orks there are, the more kunning they get. This is again why you don't want to kill them slowly.

Orks in melee combat get stronger and faster and more killey the longer the combat goes on. Even tyranids tend to lose, according to fluff, if they let the orks get their choppas wet. They are literally 'unstoppable once they get going', which is why everyone goes out of their way to not let them get going. And that's not in terms of just numbers, it's in terms of individual combat potential.

Orks live everywhere. Everywhere. Sure, some land on the zerg planet and the zerg kill them all fast because they are apparently awesome and know all about orks, but there are now orks in their asteroid belt, and orks on all the other planets in the system, and orks in huge roks circling the sun and some other orks on their way to other systems, and some orks in functioning kroozers warping around the place on tradin mishuns or explorin' or whatever else.

If you're going to look at a grand conflict like this, you have to look at the fluff of the races and not just their on battlefield stats. What generally makes orks stop fighting is when every ork dies which whole races have specialized UNITS for doing (the Codex Astartes apparently has a whole section on how to fight orks without just making more orks), or when they actually bugger off and leave because the enemy 'aren't fightin' fun like'. Literally, if a conflict is good or fun, orks will just keep popping out of the ground and anywhere else nearby to keep the fun coming.