Background: Recently I've been reading about the Roman Republic, specifically about its last century of civil war and disorder, and I've noticed that some historians say that the Republic fell because its institutions were prepared to handle only a small territory, not the big empire that the Romans conquered throughout the centuries. The most recent book where I found this line of thought was Mary Beard's SPQR, but I've seen it in other places before.
The problem is that when these books talk about the end of the Republic, they in general just list events concerning the Gracchus brothers' fight for redistribution of land, Marius' and Sulla's civil wars, and events concerning the First Triumvirate — they never tackle the issues of obsolete institutions directly. Even the article in The Cambridge Companion to the Roman Republic pertaining to the topic only mentions this obsolescence and says the idea is correct, but doesn't elaborate on it. From these works I've gathered that Roman institutions were obsolete precisely because they permitted the existence of this kind of conflict, but I still don't understand why that's the case, which institutions were obsolete and why.
Question: What institutions were so obsolete as to cause the fall of the Republic, and why did they function properly only with a small territory?