It occurs to me that, although our fear of a threat increases as the threat grows larger, that fear doesn't increase evenly. In other words, though a threat's size may increase at a steady rate, our fear will not increase at a steady rate. There seem instead to be junctures at which our fear spikes.
Let's say you see a large spider, and your fear response is at level "X." Then you see a spider twice as big, and your fear response is at "2X." And you continue to see bigger spiders, resulting in bigger fear responses. There seems to be a size at which your fear response will climax, followed by another period of gradual increases commensurate with the increases in size. So as the sizes increase, your fear might look like this: 1X...2X...3X...5X...4X...5X...6X...9X...7X...8X...
I think this is the result of Hick's Law, a psychological phenomenon whereby decisions tend to take more time when there are more options. The greater the number of choices, the longer the decision takes. The relevance of Hick's Law to threats is this: when dealing with a lethal threat, indecision can be fatal, so threats that straddle two categories are more dangerous than threats that occupy a single category, since threats of the former kind force us to choose between two different defense strategies. Returning to the spider example: up to a certain point, increases in the spider's size will only increase our efforts to kill it in the conventional way (by smashing it). But beyond a certain size, we'll need to change tactics (treating it the way we might treat a rapid dog, perhaps). After all, you can't smash a 40 lb spider. Now, the point where we realize a change in tactics may be necessary is the point at which our fear will spike. This is because that juncture confronts us with the possibility of fatal indecision.
Here's the relevance to fiction: when devising a threat, it's useful to place the threat at the nearest juncture between likely categories. So if you feature huge spiders, make them slightly too big to be smashed, but slightly too small to be fought off with a chair. If you feature an ogre, make him slightly too big to defeat hand-to-hand, but slightly too small to easily hide from. Strive to find that "in between" weight class which will make it difficult for the hero (and therefore the reader) to choose a defense.
This principle isn't limited to physical size. It can apply to any force of antagonism: the amount of a debt (sell your house, or declare bankruptcy?), the spite of a spurned lover (change your phone number, or carry a gun?), the length of a likely prison sentence (serve the time, or flee the country?), etc. The key is to prevent your reader from settling on a solution.
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