I don't see much point in comparing the generosity of these individual people. There are a million factors that contribute to each individual's decision to give.
I think the important thing to note here is momentous shift that has happened in our culture. For all of human history, riches were something to be hoarded. Whether a king or a Rockefeller, your primary goal was to build wealth and pass it down through the generations of your family. Not that there weren't generous kings or Rockefellers, but their generosity was usually inconsequential to their overal wealth, and was almost always of a local focus (libraries/museums and other monuments to themselves that would benefit their immediate friends/decedents).
In our own lifetimes we have seen this idea turned on its head. The technophilanthropists of the modern era have breathed new life into the culture of the ultra-rich. Today, if you are fortunate enough to become
that rich, you find a project to tackle. A big project, something that others may have deemed impossible or impractical. Whether it be making life multiplanetary, eradicating malaria, or a war on child hunger. A billionaire without a foundation is an old-school billionaire (and quite-frankly, a jerk).
What an amazing force for good these men (and women) have become. With their resources (money, intelligence, networks) I doubt any problem plaguing humanity is safe. Buffet and Gates get a lot of the credit for effecting this change.
In full disclosure, I'm not smart enough to come up with this view on my own. Suggested reading:
Abundance: The Future Is Better Than You Think