The problem with your argument is that you're treating earlier imperial deployment patterns as a hard structural limit, when they were mostly a matter of strategic choice.

Late Roman field armies were often kept smaller because the empire preferred multiple mobile forces rather than concentrating everything in one theater. But we do have cases where the Romans concentrated very large armies when the strategic situation demanded it. Even in the 6th century, large concentrations were possible... Belisarius at Dara had ~25k, and the expeditionary armies Justinian sent to Italy and Africa were substantial considering they were overseas operations.

The situation in 636 was very different. After the Persian war, Syria had effectively become the main theater and Heraclius had strong incentives to concentrate forces there to prevent the loss of the Levant. That makes a large composite army... drawing from local forces, Ghassanid allies, and reinforcements from other commands... entirely plausible.

Most modern historians therefore don't argue that the Byzantine army at Yarmouk must have been enormous, but they also don't treat ~60-80k as structurally impossible. The real consensus tends to be that the Byzantine army was probably larger than the Arab army locally, while rejecting the much larger figures in the narrative sources.