I work in libraries, where we don't tend to work with these format sizes - that is, we don't use the same terminology as the book trade. Can't say I've noticed a prevalence of any one size over the others, though.
What I have noticed, however, is the steady increase of (mainly fiction) produced as 'trade paperbacks' - paperbacks with dust jackets and end flaps. I wonder whether there's any more profit in it for the publishers/printers, because I can't really see any advantage. In fact, when it comes to library servicing, they're a nightmare.