Leinad - good find!
The assumption is the PSF is maintained for all unsaturated star images...that's what I'm finding...doesn't address saturated stars?
Maybe the attached write-up will give some ideas of my data/ thoughts...
In the meantime - an analogy...
Point a laser beam at a piece of graph paper on the wall (-CCD/pixels).
The beam illuminates a patch 2 x 2 squares centred on point "A" ( - Airy disk)
After five minutes, the laser still illuminates only the 2 x 2 sq, and the intensity recorded at point "A" will have increased.
There will be no illumination recorded outside the 2 x 2 square.....
More realistically:
Same laser but supported by a shakey table, still centered on point "A" but can wobble around and illuminate up to a patch 10 x 10 squares (-Seeing conditions)
After five minutes, point "A" will still record the peak illumination and the spread will not exceed 10 x 10 squares.....
Why then would a star image appear to spread to say 20 x 20 squares??????
Other than sensor reflections etc - the feeling seems to be that the image saturation will cause the 7% residual light in the "1st Airy ring" to contribute to the image size. This doesn't seem to match....
(BTW with the Hubble...if there's no atmospherics to "smear" the Airy disk --why do brighter stars still look larger??
)