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 natural and inevitable effect of immoderate greatness. Prosperity ripened the principle of decay; the causes of destruction multiplied with the extent of conquest; and, as soon as time or accident had removed the artificial supports, the stupendous fabric yielded to the pressure of its own weight. The story of its ruin is simple and obvious; and instead of inquiring why the Roman Empire was destroyed, we should rather be surprised that it had subsided so long."
Gibbon is making the argument that the elements which allowed for the greatness of Rome could not be the forces which sustained it. The implication is that the rise to power and the judicious exercise of it are not one in the same. This allows for a stronger understanding of why the Roman Empire was, after a point, unable to be sustained. The natural state of expansion caused it to be untenable and nearly impossible to defend. This would be an example of collapsing under its own weight. At the same time, questionable decisions on the part of its leaders differentiated it from the sound and wise leadership of those who allowed Rome to be great. At the same time, the belief in its own greatness, something that impacted its ability to be reflective enough to embrace change, is what prevented the empire from modernizing and advancing.
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.