The intention of art, music, mathematics, literature, philosophy, science, etc. are to simplify ideas into simpler terms. Surely, a large scale formal work has its place and can demonstrate the one's prowess as a composer. But, for me, simplicity is another form of genius. Why write 20 pages of music, if all your musical intentions can be summarized in 8 pages or shorter?
Music deals with understanding and communication. In order to simplify, you have to understand the intentions of the music, spend the time, and delete redundancy. In order to communicate, you have to use your creative powers though compositional tools: dynamics, counterpoint, rhythm, etc. Talent is the glue which keeps it all together, and allows this to happen.
In writing, expository composition deals with expression through minimal verbosity. Composing a short work doesn't necessarily denote (pardon the pun) a less formal work, or make less powerful musical statement. For example, in the entire piano literature, I have yet to discover a short piece of music that makes a more powerful musical statement than Chopin's Prelude No. 20 in C minor. The concept of understanding and communication on part of the composer is sublime. Chopin says everything in just 3 lines of music what it would require an entire mass by other composers. And the real tease about this piece is that he already establishes the essence of the music by the 2nd line, so he could have wrote it with just 2 lines of music if he wished...
This quote sums up my point: I didn't have time to write a short letter, so I wrote a long one instead. -- Mark Twain
As always, Mr. Clemens - Twain sums up the problem so succinctly. (Come to think about it, why does succinctly have so many "C's"?
As 88man says at the beginning "The intention of art, music, mathematics, literature, philosophy, science, etc. are to simplify ideas into simpler terms" some times the simpler terms take 10 - 20 minutes. Bernstein, in his Joy of Music
followed the Beethoven's compositional process for the first movement of Symphony no. 5 using his notebooks. In most respects, it was a process of editing out non-essential materials (much as a good writer is really a re-writer). In the end, it is difficult to think of any note or measure of this work that could still be left out.
Schubert, in particular, was not the best self-editor and re-writer in his longer works. That is possibly a reason for so many unfinished works. He ran into compositional problems and set it aside, but his mind was streaming with musical ideas (particularly melodies) that he just went to the next compostion. But, he often excelled in his shorter works, such as his lieder, because his melodies had such a natural succinct quality.
An important aspect of the arts as opposed to the sciences is the aesthetic idea of balance and proportion, which is a quality that cannot be put directly into scientific terms (at least until the "theory of everything" is finally proven). Granted, it can possibly be equated to the idea that "for every action there is an equal and opposite reaction." For example, the classical sonata form is built on the principle that for every moment that the piece is NOT in tonic (particularly on the sharp side of tonic) there needs to be a balancing moment in tonic (or on the flat side -- thus the sub-dominant key area can act as an offset).
(I would make this shorter, but to quote Mr. Clemens-Twain "I don't have time...")