Why do orcs and goblins get so many troops and is there any way to do the same? Also why do dwarves have so much attrition in mountains?

Im playing as Thrars people in the Yellow Mountains and im trying to rebuild the Empire, but ive run into a problem across two runs. My characters have amazing stats, ive got great counselors, and ive got a solid retinue of 2,000 elite soldiers. I also just recently founded a holy order which gives me another 1000 or so troops, on top of that my finances are pretty good. I could probably afford to double my retinue and still be solvent during war.

The problem is that my primary target, the Goblins holding the other half of the Yellow Mountains, despite having a smaller realm than me, have around 3x as many troops as I do, and their forces seem to jump up by a couple thousand every time I look at them. When we first fought about 20 years prior, they had about the same numbers as me, it was 6,000 against 7,000 including some mercenaries boosting my numbers at the time. Then the moment the war started their numbers jumped to 12,000. I was still able to barely beat them due to superior troop quality, but every battle was expensive on my levies which didnt replenish at even a fraction of their speed. I was able to kind of beat them, but not enough to achieve my war goals, mostly because of utterly insane attrition I was suffering any time I went on the offensive which didnt seem to even inconvenience the goblins when they entered my lands.

Since then ive grown my armies to where even without Mercenaries, we've got about 9,000 troops. The problem is that in that time, and without any meaningfuly territory acquisitions, the goblins have gone from 12,000 to 27,000, I know I have them beat in quality, but at least in my experience theres just no way to beat that kind of numerical difference. I am not allowed to try to make any alliances with my neighbors because, apparently, none of my neighbors border my lands, and relying on the AI to help during war basically never works out for me. They always go off to some corner and dont do anything till its way too late.

Ive sent an expedition to the Western Yellow Mountains to try and expand there, but theyve got their own goblins to deal with and who knows how long it will be before they become a vassal, certainly not me.

Im wondering if anyone can help me understand how the goblins got so many troops so suddenly, and if theres an upper limit, as well as how I can counter it or if I can use this same black magic for myself? Right now the only viable strategy i can even think of is waiting for 20 years to build up a horde of wealth and buying every mercenary order available, because spending 20 years building up my holdings is not working