Now A Medium Motion Picture

Why is it that every book ever transformed into a movie is billed as major?

"Now A Major Motion Picture" gets slapped on the cover -- a cover destined to be redesigned with a fetching still from the movie, just as soon as the publisher can get the reprint into the works.

Who first decided that their movie-of-the-book was "major", I wonder? And how and why did it become so widely adopted that it eroded into banal idiom?

Even my battered 1960 Penguin paperback of Room at the Top bears this strapline, pasted in just under Laurence Harvey's dimpled chin. This edition came out about a year after the movie. So even in 1959, books made into movies were automatically "major".

And while we're on the subject - who, other than the MPAA, calls them "motion pictures"?