“Why not make a long odds bet with a wealthy counterparty or use high-risk derivatives to get a chance at making the large donation? In principle, economies of scale like this should always be subject to circumvention at modest cost.”
Yes, as Paul Christiano writes,
In principle some opportunities might only be accessible for big donors. $1B might go more than a thousand times farther than $1M. But if we are really only interested in expectations, then we can always just take a 1000:1 bet and turn our $1M into a 1/1000 chance of $1B. This guarantees that there are no increasing returns to money.