stretching vs expansion

stretching and expansion both are nouns.

Word	Noun	Adjective	Verb	Adverb
stretching	Yes	No	No	No
expansion	Yes	No	No	No

As nouns, expansion is a hypernym of stretching; that is, expansion is a word with a broader meaning than stretching:

stretching: act of expanding by lengthening or widening
expansion: the act of increasing (something) in size or volume or quantity or scope

Other hypernyms of stretching include enlargement.

stretching (noun)	expansion (noun)
exercise designed to extend the limbs and muscles to their full extent	the act of increasing (something) in size or volume or quantity or scope
act of expanding by lengthening or widening	adding information or detail
	a discussion that provides additional information
	a function expressed as a sum or product of terms