Thats partly it, but its not just the shorter format, its the ideas - Ballard was pretty ingenious at the start of his career - loads of ideas (check out these if you havent got em:
http://www.ballardian.com/biblio-jg-ballard-the-complete-short-stories) which he then whittled down to one or two and then repeated for the rest of his career. I would still say that he basically wrote one book with variations. Ambivalent hero crashes into utopian/transgressive project, usually through the death of a friend/relative, flirts with participation, shags the girl and eventually (mostly inadvertently) brings about the project's demise. The apocalyptic phase was basically a trilogy of his first 3/4 books, the drowned world, drought, the crystal world, and (arguably) the wind from nowhere. Concrete Island and the Atrocity exhibition are the only real exceptions to the formula after 66.
Dick had two periods of intense creativity. The early 50's when he was a struggling writer and came out with dozens of short stories which he basically rewrote (with a few exceptions) as novels until the late 60's, then from the less prolific (but more creative) late 60's to early/mid 70's (when he had his epiphany) culminating in VALIS.
@Padraig - there's space there now...