Answer:
Large states are always complex and difficult to manage, simply because of the sheer size of the geographical area that they command, and the diversity of economies, societies, and cultures that are kept under their rule.
For the Western Roman Empire, managing such a large territory became increasingly difficult, and this was one of the reasons it fell in 476.
Emperors lost control over the the farthest regions like Britain, and economic resources became more and more scarce.
The Western Roman Empire became distressed by continuos barbarian incursions, mainly in the borders (limes) of the Rhin. These barbarians became increasingly difficult to control, and with time, they took up large tracts of Roman territory, ultimately leading to its demise in the fifth century.