My eyesight without glasses is bad enough (nowadays) that I can't do a fair comparison of uncorrected vision between dark and light (can't read anything either way), but letters on dark background get significantly more blurred than with a light background. The overall room lighting level also seem to play a large role. Hence, pupil size.

My guess is that not only myopic individuals, but people with any visual problems due to their corneal shape. Also LASIK patients with large pupils (but that's to be expected).

Again, not a doctor. I am hoping some doctor will step in and comment :)