To be fair, there's not only a huge difference in scale, but I was pretty sure there was a 20k polygon limit that was bypassable if you separated stuff into subobjects in HW2.
There's a polygon limit, but it's on the higher end of the spectrum (more like around 80k) and I believe it was a problem with CFHodEd (the fan-made tool we use to import ships), not the engine or file format itself.
I managed to get the Solaris in by cutting it into only two subobjects (not counting turrets).
HW2 does have a limit; you can have a 50k poly fighter, but how many of those can you have before it lags the crap out of a computer.
A lot, as long as it doesn't have tons of hugeass textures and uses a low-poly enough collision mesh.
Honestly, HW2 is remarkably scalable. Perhaps more so than FSO.