Ok, I think I figured out the problem, with Kazan's help.
I took my test scene, which had models accurate to the meter, and scaled them down 20x. Then I took a 45 degree (~43mm) camera, and walla - the Deimos and Fenris looked exactly the same as they do in-game - same perspective, same apparent size, everything. (I'll post a screenie in a few minutes.)
So basically, FS2 renders "meters" as 20 times smaller than they should be. While this does not affect ship interaction or anything, it does affect the simulated FOV/lens diameter.
So I scaled the test models back up 20 times, and suddenly it was almost realistic. The problem was that the camera made things seem too close for the distance, like it was zoomed in. So I changed the camera to a 35mm (FOV 54), and finally, the 747 looked like I was standing there, in person, looking at it.
So now the question remains: is there any way in the code to deal with this 20x scaling issue? Because if we can do that all in the code, without having to reconvert any models, then that's 90% of the problem solved right there. The other 10% would be setting the default FS2 FOV to be 54 instead of 45.