I'm running XnView 1.96.2 on Vista.
It appears that when a JPG image has any metadata, it displays much smaller than it should in browser mode.

As you can see, all of the images are JPGs. And those without metadata display properly. Those WITH metadata, however, are much smaller than they should be.
I used the "clean" function on one of the images that displays small in the attached picture, and it displayed at its proper size immediately after the clean operation.
This problem has been around for a while.