I swear I'm not being diplomatic about this, but the answer to that is 'It Depends. Smiley versus Karla trilogy had me in raptures; I nearly cried (with misery) when I got sent the original version of David Copperfield. Both were bricks; and much as I love Dickens, there is something daunting about a thickset copy with font 8 on a Times New Roman (or something similar).
Thick books (strictly excluding text books) are usually fun. Look at A Suitable Boy; I bought it five years back and I'm still ploughing through. I haven't had the time to finish it or get bored of it (but then I'm a serial book-shifter*). And then there is The Old Man and the Sea; it is a pamphlet of a book, and I haven't (or rather couldn't) finished that either. So it depends on the content of the writing. I'm not saying family sagas are more fun or intellectually more stimulating than, well, an old man fishing in the sea, but somehow, the lack of activity in the latter seemed to deaden my interest in the book. To prove my point, look at Of Mice and Men; its near-about the same size as The Old Man and the Sea and it took me one sitting on a lazy summer afternoon to close it. As an introduction to Steinbeck, I couldn't think of a better way to start (more on that in some other post!). Content is the key. And of course, to a great degree, the reader's propensity to capture the content; though a self-understanding of the same comes only from practice.
Then there's the pace. Lee Child's books only look heavy, same as with Dan Brown, but they really aren't. They are nice and catchy and racy and all that gets to the nerves and makes one turn pages like there's no tomorrow (the stories are also mostly based along similar lines). On the other hand, one has Amitav Ghosh; you simply cannot rush past his words. Each sentence is a story in itself, that has to be chewed and digested before moving beyond the full stop. The content is much heavier and thought-provoking, and hence, despite being similarly placed in terms of physical heft, Ghosh's books take significantly longer and more patience to read.
Of course, the font size is another practical aspect. In all honesty, when Harry Potter and the Cursed Child hit the stores, I was internally chuckling with delight at the decent thickness of the book, imagining all sorts of mind-numbing complications and a sleepless nights, but then, it was truly a disappointment. The formatting of the book was more along the lines of a primary school text book - widely spaced, well-sized fonts, with considerable spaces between consecutive dialogues. Really ? What could have been possibly lost - from a reader's point of view - if it had all been spatially condensed along the same lines as the predecessors? It would have saved space in my bag and hurt my shoulder less.
Biographies, by nature, are meant to be quite thickset. And most of them are extremely interesting, since more often than not, they deal with highly public figures and the lesser known details of their lives (at least the way the subject - if alive - would like the reader to know the same). Which brings me to the conclusion, that fact is stranger than fiction, Unless, one has something as preposterously imaginative as the world of the Middle Earth or the Hogwarts or something out of Asimov, it must be really difficult for fiction based on the existing universe to hold its own for long. Human lives and real-life incidents - perhaps by virtue of their having happened already - generate more curiosity than a make-believe world. That cannot be entirely fitted into a book, however thick it gets, but it barely ever gets dull, unless, of course, there is some deficiency in the writing in itself.
Big books aren't scary but aren't always an indication of good things. Unless one is heaping scores, the intent of the writing and the reader's acceptance for the same (based on prior experiences) would be a reasonable guide to start on one. Sometimes, an affiliation to a particular author would force the readers to gobble up anything and everything - thick or thin - that she/he writes. While that is a very good practice in my opinion, more often than not, not all works turn out the same. Then there's peer pressure and sudden bursts of resurfaced passion about an author or a particular story. There are people in this world who still cannot fathom the craze behind Harry Potter, and several more must have begun digging through the memoirs of Sherlock Holmes only over the last few years. In situations such as these, the thickness of the books mostly becomes secondary; the aim is to finish it before the next round of discussion about the topic starts, lest one is reduced to a mute listener. There is absolutely no need to scorn at this habit - I have seen many a so-called culture-buff snigger at such behaviour - after all, this is what culture is all about. From a movie-making point of view, size of the story is a non-issue. So if an enthusiast, unwittingly, embroils herself/himself into a lengthy book, it really is to the reader's advantage. I had bumped into Sebastian Faulks (Birdsong)) that way, and much as I loved the book, I still prefer the BBC One adaptation which introduced me to the book. But Faulks was a find that I am very grateful for.
Size, thus, need not be a yardstick while reading (or anything in life; though, personally, I've only gotten round to reading), for the love of reading is pretty much blind and weight-inconscious.