The Roman army of the fourth and fifth centuries was probably larger than at any point since the warlord-era of the Late Republic.
And so was the tax and manpower base of the Empire, too.
Which are reasons why Peter Heather has argued that the Rome of the Julio-Claudians would have been harder pressed to hold off a Hunnic invasion of Europe than the late Empire. The only offsetting advantage is that the Germanic tribes driven into Roman territory by a Hunnic invasion would be less well organized and armed than they were in 376-455.