I've noticed this as well. My [mildly informed] guess is that FCP Studio is able to effectively copy the render and source files into a quicktime container and then flatten that into a single quicktime file. So it's only a matter of copying files where a render has taken place. Recall that in FCP there is an option to "Recompress all frames."
PPro, on the other hand (and most other apps for that matter) chooses to recompress everything. Or, if you will, "Recompress all frames" is always turned on.
Another issue is that preview renders sometimes aren't at the same quality/format as the final output, but I'm still not sure how that part works....
Ben Unguren
Motion Graphics & Editing
http://www.mostlydocumentary.com
