I believe I saw this argument once on the internet, that in the early Republican-era Roman Italy had a massively larger population than Iberia, Gaul or Britain, but that centuries later those regions had grown in population to the point where they could no longer be administered effectively or militarily controlled by Rome.
Is this a legitimate argument put forward by any scholars, or just something random I read on the internet?
The collapse of the west is a messy and complicated subject with numerous causes, but I've never seen a scholar attribute it to an expanding population - unless you count invading Germanic tribes as 'population increase'. So I'd put this down to 'random' and wrong comment on the internets.
If you wanted to pin a major reason the west collapsed (following Goldsworthy) is their inability to solve the succession problem. Rome wasted countless armies in internecine civil wars that cost them manpower and destroyed their own countryside. Those lost armies weren't available to hold the borders and keep the barbarians out.
This is a follow up question:
I heard a large part of the reason was actually under population. There weren't enough citizens in the provinces, especially along the frontier, to recruit soldiers from. Therefore barbarians had to be settled there with varying degrees of success.
Is this in any way true or am I woefully misinformed?