After thinking about this some more, I have a new insight.
I would propose that in general, the quality of a blowjob is largely dependent of the level of arousal and interest the woman has in pleasing the man, in the actual "Performing" of the act.
I can't accurately speculate for all women, but I would also propose that in general, many women are more aroused by the visual and physical stimulation brought from seeing (and holding, feeling, etc.) a larger penis as opposed to a smaller one.
If we can agree upon these two ideas, then if a woman see's a larger penis and is thus more aroused, then wouldn't she be more inclined to have more interest in performing a quality blow?
There are many other factors that come into play, and a woman can become aroused by a number of different things, but I would say that in general if the man has a larger penis, he would have better chances of receiving better quality blowjobs.
My logic is not scientific by any means, but it doesn't seem like too much of a stretch to me to assume this.