'Any army marches on its stomach'.Napoleon so said.Can it there fore be said that the fall of Rome is as a result of over expansion?
Hadrian was aware of this problem and drew back from some of the less defensible outposts conquered by Trajan to consolidate. It has also been said that the economic returns from Britain never really justified the expense of its occupation. However I think part of the question is not so much why the Empire fell apart but how it held together so long, given inherent instabilities.As Emperor you had to control the political centre at Rome, the grain supplies from Egypt and the military power bases along the Rhine and in Britain so revolts often took the form of miitary revolts or Egypt disrupting the grain supply so that the Emperor had to go off to deal with one bunch of rebels only to find the next lot attacking Rome.
I belive that it was indeed the fact that the Roman people of power got to big for their togas.