Slaver ants attack strong, not weak colonies

If you're an ant and your anthill wants some new slaves, you attack the weakest colony you can find, right? Weirdly, no. German researchers found that one slave-making species they studied picked difficult targets, even thought these would inevitable result in more casualties among the attackers. Why? For Protomognathus americanus, which has small colonies of its own to begin with, it appears that losing scouts probing a lot of possible targets is a serious matter. Making fewer raids on nests with more pupae to steal is actually the lower-risk tactic for the colony as a whole.
COMMENT: It would be interesting to look into how this compares to the historical tactics of human slavers: Arab and African slavers would attack whole villages to get captives, either for their own use or to sell to Europeans and Americans. Did they pick bigger targets, like the ants?