As noise is typically highly amplified in shadow OOF regions, these are the sorts of images that will take up more card space. Images with high ISO shot in good light, such as for high shutter speeds, will not increase the file size as much as images shot in low light at high ISO with large amounts of dark OOF areas.
Not true. Are you suggesting that the signal in dark areas of an image has a selectively higher gain, or something like that? I don't buy that. Read and thermal noise are constant across the sensor. In brighter areas of the image, shot noise dominates and is added to the read/thermal noise, meaning there is actually more noise in bright areas.
A quick empirical test demonstrates my point. I set ISO 25600, f/10 in Av, shot one in-focus shot out the window (bright overcast) which metered at 1/6400 s, a second OOF shot inside a dark closet which metered at 0.3 s. The outdoor shot size is 26.3 MB, the dark closet shot size is 30.3 MB - as the hypothesis predicts, the bright shot has more noise.
I think the explanation for your statement about more noise in dark, OOF areas is not technical, but psychophysical. The human visual processing areas try to form patterns, even where none exist (stare at a dense, regular, repeating geometrical feature and you can observe this). In the bright and/or in focus areas, your mind pays attention to the subject(s), ignoring the noise. In the dark OOF areas, there's nothing else to pay attention to, nothing to mask the noise, so you perceive it as amplified.
It is the latter situation. Yes.
That's a relief, thanks for reporting back!