Regarding late Sasanian army sizes...

[Blatta Optima Maxima]

How is it possible that, when fighting the Byzantines, Persian armies rarely numbered more than 60 thousand troops excluding siege workers, but the modern consensus about the Battle of al-Qadisayyah is 180 thousand troops?

[Gäiten]

I can not believe that is a fact. Your source for that?

[Blatta Optima Maxima]

Wikipedia

No, but seriously, how large an army could they have fielded facing the Arabs?

[Blatta Optima Maxima]

Well, seeing as this vaguely fits the topic, why did the empire have such difficulties subdoing rebels? The Armenians fought for 20 years in defense of their faith.

[Pompeius Magnus]

You mean rebels related to the sasanian empire?
Well, it depents once more of the geographical structure. Armenia had of course low-lands with bigger cities, however, Armenia was also a country with high mountains and cliffy and bumpy regions. The next screen shows Armenia:

Spoiler Alert, click show to read: 

At the moment I'm reading the newest Biography about Anastasios and the civil war against the mountain people of the Isaurians which lasted many years - even if they were completely outnumbered. The war lasted so many years and was one of the unknown extremly bloddy wars of the very late 5th century.
The Romans said that the Isaurians were barbarians - but this was pure propaganda to justify the war - actually it was a civil war.

The Isaurians were no barbarians at this time anymore and many of them served in regular Roman units before. They used the mountains for tactical retreats and came back at night for fighting. The same was made by the advanced community of Armenia. And when they (the Armenians) were forced to accept a battle in an open field then they still had their heavy cavarly - which is an advantage the Isaurians never had.
Another important factor is the affilitation of those people. The nation building for many settled people was far more advanced compared to the centuries before. They were defending their own country - therefore it was possible (my opinion) to deploy quickly serveral thousands semi-nobles with their Comitatus (or Azatani).

[Blatta Optima Maxima]

The battle of Avarayr? It was a field battle in which the imperial troops sustained massive casualties, compared to the rebels.