I expect it depends on the genre and, in some cases, the subgenres. SF/Fantasy/Horror, including the YA offerings, still look fairly hefty to me. Chick lit, on the other hand, looks fairly slim -- probably dieting. Romances in the popular Harlequin/Silhouette lines and the like have always been skinny. Thrillers and mysteries look to be in the middle. Literary and mainstream novels? Middish to thinnish to my taste, these days.
I myself prefer long books, and my natural writing bent is for the long short story and the chunky novel, or novel series. I think it was C. S. Lewis who said he couldn't get a novel too long or a cup of tea too big, and who was proving it at that moment by reading
Bleak House and drinking out of an enormous tub of a mug. (If it wasn't C.S., let me know who it was, so I can give him or her a mental shake of the hand.)
Bleak House, yum. Too bad it was so short.
Ideally the length of a book is entirely dependent on the story that needs to be told. Publishers do worry about the price of paper, though, and are widely said to like weight-conscious submissions from all but proven sellers.