When we worry about the problem of evil, we tend to worry most about particularly horrendous evils. But consider the atheological inference from the occurrence of an evil to the claim that there would be no good enough reason for God to permit this evil. Is this inference any stronger in the case of a horrendous evil (e.g., genocide) than in the case of a trivial evil (e.g., uncomfortable pebble in shoe)?
Here is one possibility (which I got from Trent): Say that an evil is in principle unjustifiable iff it is not permissible to allow the evil no matter how great a good would be lost or how great an evil caused by preventing it. Obviously trivial evils are poor candidates for being in principle unjustifiable. So if the inference is: "E is in principle unjustified, and hence God has no good enough reason to permitted", then the argument is indeed better off in the case of horrendous evils. Maybe there could be in principle unjustifiable evils. For instance: "everyone always suffering horrendously despite everyone being innocent". But none of the evils we observe are in principle unjustifiable. For instance, let S be the mereological sum of all the human sufferings. Then, S is not in principle unjustifiable, because it would be permissible to allow S if the only way of preventing S would result in a hundred planets full of aliens each suffering in ways comparable to S.
Apart from in principle unjustifiable evils, then, is the atheological inference from horrendous evils better than that from trivial evils? I don't know. On the one hand, allowing a horrendous evil requires a much more potent justificatory reason. On the other hand, many horrendous evils do very obviously bring along with them the opportunities for very great goods--exercises of courage, compassion, patience, forgiveness, etc. Granted, many will query if the value of the opportunities would be sufficient to justify God's allowing the horrendous evil, and intuitions will differ, but at least we can, typically, point to a number of uncontroversial goods of quite high magnitude. On the other hand, with the trivial evils it can often be harder to point to any goods (e.g., consider the uncomfortable pebble in one's shoe).
So maybe it's not harder to say in the case of the horrendous evil "There is a justification, but we don't know what it is" than in the case of a trivial evil. But if so, then why is it that when we worry about the problem of evil, we are more apt to worry about greater evils than smaller ones?