The thing that people aren't quite twigging onto for demon numbers is the fact that Malfeas is an unbelievably unstable and deadly place for that population, whatever actual size it happens to be. Beyond the normalized violence and predator-prey interactions between standard demons, or the callous rampagings of one yozi or another depopulating incomprehensible swathes of his shells at one time, Malfeas also has a tendency towards straight-up sending them colliding into eachother because he can, typically crushing everything without the smarts, freedom to travel or forethought to evade the catastrophe as a planetary extinction-level event. The Demon City's population might be astronomical in-theory, but so is the potential bodycount of even one of these, let alone the fact that such maliciously-motivated die-offs are a known regular habit of the primordial.
Because the Demon Realm is quite literally Outer Space as far as the vantage point of Creation is concerned. Demons are the weird and curious lifeforms of indeterminable cosmic origin, existing somewhere in great numbers, perhaps doing something we can recognize as 'living,' but ultimately meaningless in passing save being visible to us at this far distance only as the slow-motion remnants of light reaching us from billions of years ago as their home galaxy was engulfed in a supernova magnitudes larger than our ability to comprehend. Perhaps if we ever met one, spoke to it and learned how it lived and thought, things would be different, but for now they are afterthoughts as rays of gamma radiation scour whole reams of the universe barren outside of our bubble of habitable security. Each crumbled shell of Malfeas is a lifeless rock in the night sky which could have held promise, but never had the chances we did.
It makes it so that First Circles are quite literally a disposable teeming of masses, and any attempt for a sorcerer to try and form a healthy rapport with one who is not even a notable Citizen is doomed to failure when that undistinctive informant is thrown into the cosmic meatgrinder along with literally millions of its fellows. Each one might be a unique and special individual when parsed through the personal experiences of a mortal in Creation, but in the sense of the yozis greater being, they will always be eyelash mites and white blood cells. The only thing which stops any of the given species from being exterminated entirely is the stability formed by the domains of Third and Second Circles to express their natures, much like the Exalted defense/mobilization of humanity during the Primordial War, and the sheer size of the place itself working against it. Being spread across every shell insures that even the worst of the yozis self-inflicted atrocities could not possibly kill all of them.
This doesn't render Creation a less significant place any more than Autochthonia having literally millions of attendant automata-spirits managing his titanic continent-organs does, or the Underworld playing host to a population of literally every willful dead since the end of the War which spawned it. Because if nothing else, Creation is where stories happen foremost. The Demon Realm is ruled by the mad whims of the yozis, where, again like the void of space, death is a statistic and pervasive background noise to anyone seeking to derive some meaning out of it, so a story may only be found if you are willing to expose yourself to it. Such a thing renders itself insignificant by the sheer vastness of the distances and inapplicability of its nature to anything outside itself, not when there are things like wars being waged and livelihoods to be secured right here in our own backyard.
A species of endangered owl dying out from being driven out of its habitat is too big to grieve for long when you've still got electrical bills to pay, and trying to conceptualize the magnitude of the fluctuating amount of demons within Malfeas is no different.