The last couple of years, the Silverlight viewer has expanded to full browser window on normal view.aspx pages, but expanded to full screen on any embed.aspx page (this is what the hero synth/pano on the homepage uses, as well as what is used for any embedded synth/pano on other sites.
Maybe something has changed recently, but I would check to see whether the difference that I describe above accounts for your different experiences.
The main reason that the view.aspx pages do not use true full screen, I believe, is that Silverlight apps deactivate most of the keyboard keys when they are in true full screen mode. Since more people who are using the site are familiar with the keyboard navigation keys, it would be more irritating for photosynth.net regulars to have half the keyboard commands taken away every time we enter full screen. By contrast, most people who come across a synth or pano embedded on someone's blog probably don't know the keys, so won't miss them.