Books are getting fatter – and have been doing so for decades. When I first started reading science fiction in the late 1950s, the typical full-length novel was no more than 200 pages long (about 80,000 words). This remained generally true throughout the 1960s but then books began to expand, resulting in today's doorstops. Clearly, the genre has something to do with it; Tolkien set a standard in the length of fantasy novels (as in so many other respects) and it seems today that no fantasy can be regarded as serious unless the story fills up at least a trilogy. However, SF has followed the trend, albeit more slowly. The question is – does quantity equal quality? Are today's novels better for being so much longer?
I never really considered the length of story to have any bearing on it's ability to draw me into the story. You mentioned works by Asimov and Clarke, two of my favorite reads, but at the same time I'm equally as happy reading something like Cryptonomicon by Neal Stephenson.
If I find myself enjoying the story then for me an extra-long reading just means there is that much more for me to consume.
I think it almost depends on the author and the particular sci-fi genre.......one example comes to mind after reading Max Brooks' novel World War Z. After reading it, I kept thinking to myself, 'where's all the character development' and at the same time, I kept thinking to myself, 'forget that; I can visualize the characters in my head, thus I don't need any "character development" to imagine who/what the characters were. There's also the question as to whether a particular work could be adapted for either the big screen or for television and whether that has any influence on an author as they're writing; if an author thinks for a moment that their work may be on the big screen or television, that could also influence how they write. As to the length of sci-fi novels these days...I go back to what I said at the beginning of this post; it depends on the author and the sci-fi genre that their work(or works) fall into(hard sci-fi, military sci-fi, apocalyptic/post-apocalyptic sci-fi, etc.).