I was probably wrong to say that Catholics were entirely outnumbered.
The Army was, of course, in mere numbers, only a fraction of society. We are not certain what those numbers were; at the most they may have come to half a million_they were probably a good deal less. But to judge by numbers in the matter would be ridiculous. The Army was normally half, or more than half, the State. The Army was the true cement, to use one metaphor, the framework to use another metaphor, the binding force and the support and the very material of the Roman Empire in that fourth century; it had been so for centuries before and was to remain so for further generations.
Hilaire Belloc - The Great Heresies - The Arian Heresy
There were also great swaths of the Roman Empire that were lead by Arian Bishops
Arius went over from Egypt to Caesarea in Palestine, spreading his already well-known set of rationalizing, Unitarian ideas with zeal. Some of the eastern Bishops began to agree with him. It is true that the two main Syrian Bishoprics, Antioch and Jerusalem, stood out; but apparently most of the Syrian hierarchy inclined to listen to Arius.
And it stands to reason that St. Athanasius wouldn't have been made to endure such hardships for standing by his excommunication of Arius and rejection of the Arian Heresy had there not been official support behind reconciliation (which in the case of Arianism is really an impossible heresy to reconcile)
It was also the religion of the "spiritual, but not religious" types
When the power of Arianism was manifested in those first years of the official Christian Empire and its universal government throughout the Graeco-Roman world, Arianism became the nucleus or centre of many forces which would be, of themselves, indifferent to its doctrine. It became the rallying point for many strongly surviving traditions from the older world: traditions not religious, but intellectual, social, moral, literary and all the rest of it.
It was a palatable version of Christianity because once you deny the divinity of Christ, you re-make Him in your image; painting with your own palate.
And, Arianism didn't just go away because it was condemned and Arius died. It grew. So if it was strong in force, wealth and worldly power before, it had to have increased, a bit at least, before it fizzled out after the fall of Rome.
But, I'm not going to claim that there were more Arians than Catholics at any point in the history of Christendom, just more clout and prominance:
Since there is no stronger force in politics than this force of social superiority, it took a very long time for the little local courts to drop their Arianism. I call them little because, although they collected taxes from very wide areas, it was merely as administrators. The actual numbers were small compared with the mass of the Catholic population.
One might pray that the official rationalistic religion of our current governing classes be dropped a little faster.
So no, I think it might be hyperbola to have claimed that Catholics were outnumbered. We certainly are outnumbered in the USA.