Baatezu

==== I've always heard it said that there are infinite demons, but that devils reproduce through soul-larvae. If there are infinitedemons, why haven't they won the blood war? Even if a vanishingly tiny percentage of demonkind cares enough to participate at any given time, that's still a literally infinite army. Tactics only makes up for so much. ====

Demons are infinite. Devils are transfinite. Much of the rabble of demonkind forced to serve in the War is weak, useless and/or treacherous. This isn't a case of there being infinite balors versus countable pit fiends - there are EXCEEDINGLY few balors involved in the Blood War at any time, those mariliths involved are almost all wasting their time, and the nalfeshnees and glabrezu have other preoccupations that largely keep them removed from the war. In short, there's only a very small amount of contribution by true tanar'ri to the Blood War, and much of that involves the coercion of the molydei. None of the major Demon Princes care about the Blood War.

On the devil side, there's much more organization and force commitment. Bel is mandated by the Lord Below to prosecute the War on the side of Law, and has the Dark Eight working to ensure the supremacy of Baator in the conflict. Every rank from lemures up through to pit fiends can be engaged in the War. It's not only a matter of tactics (mariliths are excellent tacticians, and when a molydeus can be bothered to directly captain a war effort, there will be many dead baatezu indeed), it's a matter of sheer commitment and power level. Demons win on numbers, but devils win on force.

Don't discount the role the yugoloths play in keeping the balance, either. The Blood War won't end until someone very high-up wants it to.

(Demons also reproduce through larvae - larvae are the key to the whole of the War, used to create helpers such as imps and quasits or melted into lemures and dretches to fuel the raw numbers on each side.

The key difference is that manes can form from the Abyss as raw petitioner souls, whereas the baatezu are not natives of their plane and must hunt down nupperibos to render into lemures.)

==== Conversely, if there are a finite number of devils, but each layer of Baator is supposed to be infinitely large, what happens to all the empty space? ==== Devils are transfinite - their numbers, while not infinite, are certainly uncountable. There's not a tremendous amount of empty real estate as a result. Devils, unlike demons, also care about the empty space more. You're more likely to find an empty cavern in the Abyss than in Hell.