An interesting question, to be sure — but one that would be better asked at the height of the Principate than the period you are referencing.

While you are correct to note that Julian had gone to extraordinary lengths to muster his force — he concentrated at least a large part of the field armies of Gaul and Illyria both, along with the usual army of the Orient and Praesental guards) it is important to keep in mind that the mid fourth century empire was experiencing a deep crisis of military recruitment. While this was not on the scale of what Honorius’ desperate regime faced during the fifth century, it was nonetheless becoming a massive problem for imperial statesmen.

Julian’s invasion force was estimated to be around 60,000 men — perhaps even less. This is surely a large force, but the Republic of Rome, controlling only parts of Italy, mustered 80,000 men at Cannae against Hannibal. When that force was annihilated, they experienced little difficulty in replacing it. Every one of Julian’s soldiers was a superlatively expensive professional warrior — and while I am not of the school that disparages the capability of the late antique Comitatenses, there is no doubting that the Empire’s ability to muster armies was declining.

This was a solvable problem, but the Dominate lacked the political will to face it down — it would have required a pay raise for the army to make it an attractive career once more, and doing that would have necessitated breaking the grip of the landed elite over state finances. It is worth noting that the post-476 eastern Romans under Anastasius solved this problem with alacrity. While they did face revolts and opposition drawing on disgruntled elites, it is stunning to think that a state controlling half the territory of Julian’s empire was able to muster armies of comparable size with greater ease than the 4th century Dominate.

So now that we have that in perspective, let us consider Shapur and his state. While Shapur did maintain a force of superlative professional regiments, and his royal guard was a powerful unit in its own right, Sassanid Persia’s army shared several characteristics with Republican Rome — insofar as when the Shahanshah raised the flag, noble and peasant levies would be summoned to his position. These men were not long service professionals; many brought their own weapons and armor, and while this meant that the Sassanid infantry was nowhere near the skill and equipment of the late Roman soldier (excepting the elite Daylami foot), they were numerous and often brave. The provincial noble cavalry, however, were every bit as skilled as any Roman — and in some cases, more so.

So Shapur’s military system was more decentralized, and while this posed problems of its own it did mean that the Persians could raise numerous forces without the headache of maintaining a long-service standing army.

I will make one final point — many scholars believe Julian should NOT, under any circumstances, have concentrated that many men in one place. While some (Goldsworthy, i’m looking at you) despise the late Roman capability in pitched battles, I do not — but it is certainly true that the field armies worked best on a smaller scale. None of Julian’s commanders nor Julian himself had any experience commanding a force of this size, and it showed. Julian was a good general, but he was a good general in a late antique context.

In conclusion, let us remember that by Hellenistic standards Julian’s force was commonplace — it was huge only in the context of late antique Rome.

Sources:

Adrian Goldsworthy’s How Rome Fell, and Peter Heather’s books The Fall of Rome and the Restoration of Rome.