First, a disclaimer: my knowledge on the late antiquity in general is pretty weak. With that in mind, to the question. One thing I cannot help but notice about the Roman campaigns and battles in the late Antiquity is the smallness of the armies. Even a campaign led by an emperor usually seems to have involved no more than 20,000-30,000 troops (i. e. Argentoratum and Adrianople), with the only exception I can think of being Julian's Persian campaign.
Now, this may not be that perplexing if not for one factor: pretty much every handbook and introductory books about the late Roman empire I've read states that the size of the Roman army doubled under Diocletian and Constantine.
So, assuming I've got these two factors correct, the following conclusion becomes inevitable: Despite the overall size of the military being massively increased, the Roman empire's ability to concentrate its troops in one location has severely diminished.
The question is, why? What caused this?
The answer to this question is simple: logistics.
It's a really common misconception that when the Romans amassed armies for campaigns, they were exceptionally large. Like 100,000 men or so. In reality, throughout Roman history the overwhelming majority of campaigning armies numbered around 20,000 to 30,000 men because that was logistically the upper limit of what could be effectively managed. E.g. the army that invaded Britain was only like 20,000 to 30,000 large. The army that fought Boudicca numbered about 14,000 if I recall correctly. Corbulo's campaigns against Armenia and Parthia usually only involved aorund 20,000 men. And all of these were when the empire was at its "height" in the first century AD.
We do see instances in the late Roman period where they amass larger forces (The Battle of Adrianople 324, Battle of Mursa, Constantius II's Sarmatian campaign, Julian's Campaign into PErsia, Battle of Adrianople 378, Vandal Campaign of 441, Battle of the River Utus, Battle of the Catalaunian Fields (Arguably), Vandal Campaign 468, Anastasius' campaign with Persia in 502, etc.) But overall the frequency of this wasn't any higher or lower than it was in the Principate or even the Late Republic. It was a matter of preparation. According to the work of Roth and others, based on primary source and archaoelogical evidence even a campaign constituting the usual 20,000 to 25,000 man army could take 6 months to prepare for, and that was for the Romans who had an advanced and highly organized logistical system. Campaigns with even larger forces could take longer, but really were just more taxing on resources and accomplished within similar timeframes. It was actually mobilizing and communicating that was more difficult. An army of 20,000 men assuming it's 8 ranks wide (so a wide road) and assuming 3 feet of space between each man and 1 foot of space per man, would be 10,000 feet long, and that's just a really rough estimate off the top of my head (the 3 feet number is based on Vegetius and Polybius though). That's roughly 2 miles, not including the baggage train which would stretch for miles further. That's really hard to manage and most of the time the Romans were marching 4 wide, so the front of your column could be reaching your destination at the same time the end was leaving the camp. Now multiply that by 5 for an army of 100,000 men. So now you have to manage 4 to 5 individual marching columns, each taking a different route, each needing a guarded baggage train, and have to coordinate and communicate between them. That's really, really, really hard for a pre-modern army, even into the Napoleonic period.
It's also now debated whether or not the Roman army actually increased in size under Diocletian through Theodosius. The army of the Notitia Dignitatum seems to number around 450,000 to 650,000 men, give or take, but the work of modern authors (starting with Luke Coello) suggests that a lot of the evidence for principate era units is still missing, and that when you start tallying them up they come in the 400,000+ range, not in the 300,000 range.
For more on this the basic reading is Jonathan P. Roth's "The Logistics of the Roman Army at War."
Thanks for the question! It might be helpful to know what the late Roman army is so small in relation to; are we talking in relation to previous periods in antiquity, modern days, or different places?
Specifics would be very helpful! 😁