This has to do with the size of the ship. Ok I'll be more specific, you know how the Star Destroyer is the same size right? Well if they are both the same size why is the Vengeance's bridge window big on the hull while the ISD's bridge window is tiny on the hull. Here are a couple of pictures for reference. I highlight the bridge area in green on the QMX model.
[hotlinked images removed]
You might want to take a look at the QMx Enterprise model before you start using their Vengeance as proof of anything. As cool as it is, it's not particularly accurate - there is no bridge window and there are windows on the secondary hull and hatches in the saucer which aren't on the CG model seen in the film.
Furthermore, it's from the scale on QMx model that we get the 4800ft size figure from!
My question is this: Why would they lie?
What on Earth do they have to gain by pretending their starships are one size while really making them much smaller? How does this ridiculous conspiracy you're suggesting help them?
Oh, and here's a pic of the Vengeance's bridge window from the movie.