I can confirm that is how it is working in the latest of both Aperture and OS.
I don't remember for sure how it use to work but not upsizing seems to ring a bell.
I checked on my Snowleopard machine with Aperture 3.2.4 - it also will blow up the the exported image when I use "fit within" on an image smaller than the specified size, exactly the same as with AP 3.4.3 on MacOS X 10.8.2.
I never noticed, when this did change. You might try the Trial, to check it Aperture 3.1.2 really was behaving differently.