I remember reading and enjoying that article (this one, I think). I would think that the same argument would apply regardless of the scale of the donations (assuming there aren't fixed transaction costs (which might not be valid)). My read would be that it comes down to the question of risk versus uncertainty. If there is actual uncertainty, investing widely might make sense if you believe that those investments will provide useful information to clarify the actual problem structure so that you can accurately target future giving.