It was just the number assigned to the film format by Kodak when they first designated that format by number.
Some of those film format numbers have a slight connection with the size, while others don't (e.g. 120 and 620 - same film, different spool).
Some of those film format numbers were assigned consecutively, while others aren't.
Some film format numbers were used, then went out of production, and then the number was re-used for another format later.
Here is a Wikipedia article: http://en.wikipedia.org/wiki/Film_format