Home politics in Rome were incredibly fragile. The ruling elites were never really safe from the next angry uprising, which led to all kinds of economic and political appeasement - this is where we get the phrase "bread and circuses" because that's what the Emperor literally had to hand out for free. Whoever proposed heavy conscription would not long keep his job (or head). Italia was essentially a black hole that sucked in resources from the outer provinces - troops from Germania, bread from Aegyptius, taxes from everywhere else.

As for the Malthusian trap, for Italia at least the answer is simple: they emigrated. Joseph Tainter's The Collapse of Complex Societies goes into great detail on Rome's perverse economic/demographic situation.