I would somewhat disagree that it is extra crammed, I mean no more then any other holiday, special event.. As mentioned Dec. 25 is not a national holiday, all stores are open and things are running at regular ours, as it is a regular work day and so is December 24 and the 26th, so I wouldn't expect crowds out like you might see on New Year's eve at certain places, and even then, there's usually some mass exodus out of Tokyo during that time frame as well. Some equate Christmas to more like Valentines day where couples go out in the evening, and as mentioned you certainly have a lot of that.