expansion vs stretching

expansion and stretching both are nouns.

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

As nouns, stretching is a hyponym of expansion; that is, stretching is a word with a more specific, narrower meaning than expansion:

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

Other hyponyms of expansion include dilatation, dilation, distension, distention, inflation, magnification, extension.

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