While certainly big compared to normal insects, these dungeon insects aren't big enough to make me think they might be supernatural monsters if I didn't know about them. And accidentally getting strong enough to keep killing them as easily as one might expect of an insect that size would make it harder to jump to a supernatural conclusion.
I mean, if you just happened to kill an ant in your backyard the size of a small dog with a few wacks of a newspaper would you think it may be an otherworldly monster on the level of a natural disaster? The lack of curiosity to look up what species of ant that was is a different question.