Once upon a time, three groups of subjects were asked how much they would pay to save 2000 / 20000 / 200000 migrating birds from drowning in uncovered oil ponds. The groups respectively answered $80, $78, and $88 [1]. This is scope insensitivity or scope neglect: the number of birds saved - the scope of the altruistic action - had little effect on willingness to pay.

An alternate interpretation is that people conceptualize the problem not in terms of absolute number of birds saved, but in terms of the fraction of birds saved. And they have no idea how many migrating birds there are. Presenting the number 2000, or 200,000, probably suggests to them that that's on the order of how many migrating birds there are.