Comparing hardcover editions (via library catalog) the original is 823 pages and the 1990 uncut version 1153 pages. My guess is that in the 70's King was still somewhat restrained by his publishers. Back then publishers routinely assigned editors to work on manuscripts with authors in an effort to publish better books and authors generally respected them. These days editors mostly try and find new authors of potential bestsellers, rather than actually edit books. Once an author achieves stratospheric sales figures they are generally regarded as an 800 pound gorillas and whatever they turn in gets sent to the printer after some proof-reading for typos. So, in terms of what's different about the two texts I assume that King put back in all the adjectives, adverbs, and product names that were cut from the original edition and voila(!) a couple hundred extra pages.