There is a section at the bottom of FastPictureViewer Professional's download page where we succintly list what each release brings. Similarily, the Codec's page has a "News" section where significant releases are listed.
As with all complex software we constantly fix issues and improve things on a daily basis. New features and major fixes affecting important customer scenarios are always listed but minor adjustments or update sometimes are not, for example we often correct edge cases that very, very few people ran into or make minor adjustments, tidy-ups or tweaks that really don't deserve front page news. Sometimes, FPV Pro releases are made to keep the common codecs in-sync with the FastPictureViewer Codec Pack, with no functionality change in the viewer itself.