Books - and publishing - have always gone through phases of more common/popular lengths. It used to be that 200 pages or so wasn't unusual. It's thanks to certain very popular authors that more and more books became VERY long. One very clear illustration of this trend is in children's novels. If you take a look at the length of the fourth Harry Potter book, and the average length of children's novels that came out shortly after it vs. the average length that came out before, there's a HUGE increase.

Novellas - also called novelettes depending on the word/page count you use to define 'em - are popular right now. At some point it will change again.