I feel like there's some cherry picking going on. I mean, issues with germanic tribes and inland conquests did play a role but... what about upper egypt, paetrea, britannia, etc. The edge of the empire was the edge of the empire... with frontier problems. Why did germania fail, but not those other ones.
I mean size is an issue. Travel & messaging are issues. Empires are very much defined by these things, etc. I just don't see where the clear watershed is.
Early modern european empires colonized the americas, SE Asia, the african coast... Those were months of treacherous sailing. "Empire" in those cases meant something totally different than for Rome. Rome marched in with troops and took political control. Portugal & the Dutch, England and such established small colonies that grew in power until they were hegemons. Sometimes just a small port or town.
Anyway... I think Rome's limiting factor in germania and the european frontier was barbarism. NW europeans didn't have civilization: states, monarchies, temples, senates, roads, nations. The germans didn't even know they were germans until Romans told them. Rome's empire was based on assuming political control over existing political institutions. It's economy was dependant on this approach. Rome's African and Asian territories had been civilized for Millenia. Jerusalem, Carthage, Luxor, etc. Europe had no cities. How do you tax?
In northern europe, arabia and such Rome's empire looked like the early days of early modern colonialism. Slave export & frontier plantations supporting a Roman villa lifestyle for a small number of colonists. Most of the natives didn't pay tax, or participate in Roman life.
Egypt, under rome, shipped grain to the capital and enriched Roman officials. In "the colonies," it worked in reverse. Rome had to ship them soldiers and money to keep the colony going.