Do you recall the basis for that? As in, how that 25,000 is determined to be the maximum sized-army?
The Roman Empire of Maurice's day seems to be at a low population ebb relatively speaking (thanks to the plague in Justinian's time) and has military commitments the length and breadth of almost all its frontiers, so my suspicion is that it has more to do with a lack of manpower than a lack of an ability to support a larger force (meaning, concentrating more than 25,000 troops is more than the Roman Empire can afford to do, not a logistical concern).
Add in the devastation of the war/s after his murder/assassination, and it'd be even worse (@ Stanley).
I'm quoting here from page 100 of Haldon's "Warfare, State and Society in the Byzantine World: 565-1204".
On the Strategikon, Haldon states:
"An average force of between 5000 and 15000, and a large force of from 15000 to 20,000 is discussed, figures which coincide with those which can be deduced from an earlier 'official' document, the
Notitia Dignitatum of the early fifth century."
Haldon goes on to suggest that the total number of
Comitatenses available to the Empire at the end of Justinian's reign was probably somewhere around 150,000. Appeals from Italy under Tiberius II for more men would suggest that Italy's field armies were denuded- and he suggests also that under Phokas and Heraclius, the armies of the
Magister Militum per Illyricum were destroyed altogether, a loss of around 18,000 men. Adding in casualties of the Persian wars, plus the worsening situation in Italy and the Balkans, and I'd say of all the field armies of the Empire in the 630s, we have a figure of about 100,000.
Of course, to that we should add the
Limitanei, but we can assume they'd have been even more badly shredded by the Persian and Slavic invasions than the field armies were. There's evidence from Egypt that Heraclius made some effort to re-establish them, but this would have been a process that would only just be starting off in the 630s, so I'd guess no more than a maximum of maybe 25,000
Limitanei were present in the whole of the East at the time of Yarmouk.
That's why I think an army of 55,000 at Yarmouk ITTL is too large. Even under Justinian, we should note, the maximum sized army Procopius and Agathias discuss is 30,000 men.