People often claim that a large harm to one person is more important to prevent than a very great number of smaller harms to different people. But this "anti-aggregative" view (that we ought to prevent the one great harm rather than the many smaller ones that quantitatively outweigh it) is indefensible for the straightforward reason that repeated iterations of such a choice would make everyone worse off. As Parfit explains in 'Justifiability to Each Person' (p.385):
[W]e might claim that
(1) we ought to give one person one more year of life rather than lengthening any number of other people’s lives by only one minute.
And we might claim that
(2) we ought to save one person from a whole year of pain rather than saving any number of others from one minute of the same pain.
These lesser beneﬁts, we might say, fall below the triviality threshold.
These claims, though plausible, are false. A year contains about half a million minutes. Suppose that we are a community of just over a million people, each of whom we could beneﬁt once in the way described by (1). Each of these acts would give one person half a million more minutes of life rather than giving one more minute to each of the million others. Since these effects would be equally distributed, these acts would be worse for everyone. If we always acted in this way, everyone would lose one year of life. Suppose next that we could beneﬁt each person once in the way described by (2). Each of these acts would save one person from half a million minutes of pain rather than saving a million other people from one such minute. As before, these acts would be worse for everyone. If we always acted in this way, everyone would have one more year of pain.