Back in the day ('72) theaters here in NY were still the old, original sized ones (before they were divided up into 6 or 8 little rooms). Consider that most of them showing the original release were packed for months, and the more spectacular scenes (ie. Sonny's death, Jack Woltz and the horse's head, etc.) were followed by upwards of 500+ people going wild.

You waited for a Friday or Saturday night showing? Fugghedaboudit! The theaters had LOOOOONG lines right through the summer (and the movie was released in March).

Because of the success of Part I, the anticipation for Part II (two years later) was tremendous, and once word got out that Part II was equally as good, the lines for that movie were equally as long.