The shrinkage of the air/CO2 is only about 4% when you go from 20C to 0C. The shrinkage of the beer is much smaller. This means it is the increased solubility of the CO2 that is the problem here.
Yes the increased solubility of the wort is the problem, but that doesn't mean the fullness of the keg isn't important. If the keg is 50% full you'll have a lot more CO2 in it at 20 psi than you will if it is 99% full. The increased solubility of the wort will only absorb so much.
But as I said, if your keg is very full, as in very full
, (perhaps I should have used more emphasis) even a small amount of shrinkage can have a huge effect. Am I the only one who tries to fit 6 gallons into a 5 gallon keg?