I will have to test this out, usually your number of armies are comparable to the AIs but I have noticed Baktria rolling over everyone a bit more than usual when I try playing as Parthia.

I have experienced something somewhat similar when I let the Etruscans keep Alalia. They could field 3 full stack armies at all times, and kept sending one full stack army to attack Neapolis.

My economy could barely support 3 armies even with all of Italy under my control.

Darrick Skarr has made income modifications (especially concerning minor factions), that may help this issue but I think it is also just a problem with recruitment/mercs and the AI. I used his minor faction reduction mod in my current Parthia game; it seems to be working alright HOWEVER I found the diplomacy then changes a lot. I.E. if you have Dresden's mods, there will be a lot more confederating--or you'll find yourself in a 3-6 faction mutual military alliance with no way of taking out the others unless you wait for a loooong time / chance taking on the rest. Check my screenshots for an idea of the map: I had Aria, Sagartia, Arachosia buddy up to me so we all lived through Baktria's reign, and now Armenia is huge and allied with us... very strange how diplo changes when you're trying to fix ANOTHER problem.