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When considering star size in photography -- especially long-exposure photography -- it is important to account for ultra-violet starlight, particularly between 300-400nm.
Light at these wavelengths is not absorbed by the Earth's atmosphere (in fact, the atmosphere does not completely absorb light between 280-300nm) and is more susceptible to atmospheric diffraction compared to light at longer wavelengths. This very fact alone will cause a star to become "bloated" over the course of a long exposure.
For most observers, this light is not visible to the human eye but is readily picked up by unfiltered monochrome CCDs and indeed color CCDs not equipped with a UV blocking window.
Choice of CCD sensor will almost certainly have an impact on star size when these wavelengths are not properly restricted. For example, a KAF16803 has an approximate QE between ~25% (350nm) and ~40% (400nm), whereas an ICX694 is much more sensitive to UV light, with sensitivity ranging from ~30% (350nm) to ~65% (at 400nm).
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