Neal Stephenson is a famous SF author, best known for Snow Crash (1992), The Diamond Age (1995), and Cryptonomicon (1999). These books are respectively, 470 pages, 450 pages, and 1150 pages: his books are never thin. I think Snow Crash is one of the top 10 SF novels ever written ... and I'd probably put Cryptonomicon in the top 20. I've read Snow Crash perhaps three times, and first read Cryptonomicon while on my extended vacation in southeast Asia in 2006. Its daunting size has so far prevented me from re-reading it, but COVID-19 has got me started.
I wasn't particularly thinking about the question of why his books are so thick, but reading the following passage in Cryptonomicon answered the question I hadn't known I was asking:
Randy grew up in a college town in eastern Washington State, graduated from the University of Washington in Seattle, and landed a Clerk Typist II job at the library there - specifically the Interlibrary Loan Department - where his job was to process incoming loan requests mailed in from smaller libraries all over the region and, conversely, to mail out requests to other libraries. If nine-year-old Randy Waterhouse had been able to look into the future and see himself in this career, he would have been delighted beyond measure: the primary tool of the Interlibrary Loan Department was the Staple Remover. Young Randy had seen one of these devices in the hands of his fourth grade teacher and been enthralled by its cunning and deadly appearance, so like the jaws of some futuristic robot dragon. He had, in fact, gone out of his way to staple things incorrectly just so he could prevail on his teacher to unstaple them, giving him another glimpse of the blood-chilling mandibles in action.
Stephenson loves to write digressions. Nine-year-old Randy is a digression from library Randy digression, while adult Randy who we're currently following in the main plot line (one of three main plot lines) is in Manila. He rarely goes more than a page without taking an unimportant but entertaining detour. The plot doesn't require us to know where Randy went to university. It certainly doesn't need us to know that nine year old Randy was fascinated by staple removers. But not only do we learn that, we also find out about his stapling habits going all the way back to the fourth grade. The thing is ... these digressions almost always work. Because they're so routinely absurd and funny, they're a pleasure to read while also adding depth to his characters. Who didn't love staple removers as a kid? And that makes this funny.
This strength of his writing also becomes a significant weakness when the writing isn't quite as good. An example of that is 2011's Reamde (sic), another 1050 page epic with serious character development ... but in that book the details feels long, drawn out, and even painful instead of endlessly entertaining.
His best known works are well worth reading (arguably even "required reading") - especially if the quote above entertains you. But his other works should be approached with caution.