short subject vs motion picture

short subject and motion picture both are nouns.

Word	Noun	Adjective	Verb	Adverb
short subject	Yes	No	No	No
motion picture	Yes	No	No	No

As nouns, motion picture is a hypernym of short subject; that is, motion picture is a word with a broader meaning than short subject:

short subject: a brief film; often shown prior to showing the feature
motion picture: a form of entertainment that enacts a story by sound and a sequence of images giving the illusion of continuous movement

short subject (noun)	motion picture (noun)
a brief film; often shown prior to showing the feature	a form of entertainment that enacts a story by sound and a sequence of images giving the illusion of continuous movement