I'm not sure how much discussion this will spawn and the direction of conversation could go several ways I suppose as the question is somewhat open-ended.
I've been doing alot of personal study and reading about some of the classic composers (not classical era) and come across the phrase or the idea of their thematic or motivic material having enough weight to be usable in longer forms.
Brahms for example purposely didn't write a symphony until he was in his forties because he didn't consider himself ready. Then when he had sketches for what he thought would be his first symphony he didn't think they had enough merit to be developed into a symphony and I think it turned into a sonata or concerto.
Even on our forum a recent post by etLux had a comment about a recent composition possibly being part of a symphony but that he didn't know if the two sister pieces had merit to become such a long form.
So this got me thinking what are the elements that make musical ideas worthy of a longer form? Some composers have a germ of 3 or 4 notes while some have entire themes consisting of maybe 4-8 measures. So it must not be the length of the initial material. But what exactly is is eludes me.
Any thoughts? I'm sure there are some out there.