The current (February 12, 2012) NY Times mass market best seller list with the page count:
Bonnie Iris Johansen 384
The Girl Who Played With Fire Steig Larsson 752
The Girl With the Dragon Tattoo Steig Larsson 672
A Lady Never Surrenders Sabrina Jeffries 416
One For the Money Janet Evanovich 352
The Jefferson Key Steve Berry 592
Tom Clancy's Act of Valor Dick Couch + George Galdorisi 336
Smokin' Seventeen Janet Evanovich 336
You…Again Debbie Macomber 384
A Game of Thrones George R.R. Martin 831
The median page count is 400.
Compare this to the 906 titles that Harlequin Books and Wm. Collins Sons & Co. Canada (White Circle) published between 1942 and 1959. Not a single one is 400 pages. In fact only 2% (14 Harlequins and three White Circles) just about equal, at 320 pages, the two shortest books on the NY Time's list. Both companies worked hard at keeping books under 320 pages through cuts to the text.
Here is a current mass market paperback (at 542 pages) and one each 320 page Harlequin and White Circle.
Pocket Star - February 2012
White Circle 210 - 1944
Harlequin 217 - February 1953