One culprit can be the misguided sense that volume equals value for money. Another is the odd association between physical heft and artistic or intellectual merit – “weighty” is a compliment, “slight” is an insult.
Pretty familiar points. It's easy to be misled, in any number of directions, about what the length or left of a book (or lack of same) can signify.
These things also signal different things to different audiences. A fantasy audience tends to consider a longer book as being more "immersive" — as if "immersion" in a story only lasted when it was open in front of you and you were reading it. For a more highbrow audience, it's profundity — again, as if whatever was profound could only last as long as there was a sentence unspooling on the page.
Is Proust profound? Sure, but in a way that makes for a bad role model. It worked for him, but results not guaranteed for anyone else. You can learn a lot by reading him, both as a regular person and as an aspiring creator, but you gain very little by trying to "write like Proust".
Notes From Underground isn't very long, but there's more in it than in whole shelves of other books. Mrs. Dalloway is large and contains multitudes, even if it hasn't much of a page count. The Wheel Of Time is long, and that's about it; The Alchemist is short, and that's about it.
I long entertained the idea of writing some big Artifact of a novel, some thousand-page monster that would turn heads and insist on respect. It wasn't until I'd read a few such books, and realized how length guaranteed only length, that I realized I'd been thinking about these things entirely in terms of their external properties and not their inner lives. Anyone who writes a book to look impressive on a shelf is going about it the wrong way.