I noticed that the EA Funds often disburse large amounts of money at a time. For example, all the grants paid out by the EA Long-Term Future Fund in September 2020 were worth $5,000 or more. As a more extreme example, the Global Health and Development Fund tends to give out more than $100,000 at a time. In either case, an additional small donation of $5 seems like it would have no effect on what the EA Fund does, whereas an additional $5 donation to the Against Malaria Foundation would directly purchase one net. What is the expected marginal impact of donating a small amount such as $5 to an EA Fund?
I think the issue of marginal impact, particularly for relatively small donations going to organizations with rather large budget/granting power, is quite difficult to be confident in, and hence shouldn't be the driving factor in looking at the benefit of said donation. Much of the problem is illustrated well in Budolfson's and Spear's essay, The Hidden Zero Problem.
It does seem that many EA calculation rely on one of Parfit's 'share of the total' errors that he outlined in Reasons and Persons, making transparent advertisement about the effectiveness of individual donations quite difficult.