What is the major cause of the collapse of the Roman Empire according to this quote made by author Edward Gibbon?"The decline of Rome was the...

Basically, what he's saying here is that the Roman Empire was a victim of its own success.  He's saying that it got too big to be able to survive.

So when he says "immoderate greatness" that means that it was overly great, not moderate in how big it got.

When he says that the "causes of destruction multiplied with the extent of conquest" he's saying that as Rome conquered more and more land the causes of its destruction were exacerbated.

When he says that it "yielded to the pressure of its own weight he is, again, saying that it got too big and too ponderous.

To Gibbon, a major cause of the fall of the Roman Empire was that it got so big that it cost too much to run and it cost too much to defend.  That's what he's saying in this quote.