I have Dorothy Sayers novels that seem to be counterexamples: not that the story doesn't end quickly, but the bits about length and even focus. (At which point I note that, as peake pointed out a few months ago, The Unpleasantness at the Bellona Club is a rather good novel about the aftereffects of World War I, disguised as a mystery. And Busman's Honeymoon is, as the author noted, a love story with detective interruptions. But consider Have His Carcass with regard to length, compared to some of the earlier books about Wimsey; less cardboard and a longer book.