A low probability of large bad effects can swamp a high probability of small good effects, but it doesn’t have to, so you can have the high probability of small good effects dominate.
Let me be concrete: imagine you have a one in a hundred chance of a bad outcome of utility −100 (where if it happens all good effects get wiped out), and with the rest of the probability you get a good outcome of utility 2 (and the utility of these good outcomes stacks with how many times they happen). Then the expected utility of doing this once is 2 x 0.99 − 100 x 0.01 = 0.98 > 0, but the expected utility of doing it one thousand times is 2 x 1000 x (0.99 ^ 1000) − 100 x (1 − 0.99^1000) = 2000 x 0.000043 − 100 x 0.999957 = 0.086 − 99.9957 < 0.
A low probability of large bad effects can swamp a high probability of small good effects, but it doesn’t have to, so you can have the high probability of small good effects dominate.
Let me be concrete: imagine you have a one in a hundred chance of a bad outcome of utility −100 (where if it happens all good effects get wiped out), and with the rest of the probability you get a good outcome of utility 2 (and the utility of these good outcomes stacks with how many times they happen). Then the expected utility of doing this once is 2 x 0.99 − 100 x 0.01 = 0.98 > 0, but the expected utility of doing it one thousand times is 2 x 1000 x (0.99 ^ 1000) − 100 x (1 − 0.99^1000) = 2000 x 0.000043 − 100 x 0.999957 = 0.086 − 99.9957 < 0.
OK, that makes sense.