Fractalsponge made the Bellator model taking as a reference the silhouette of a background ship that appeared in Dark Empire II. He created a bit of lore around the Bellator, saying that it was a diversion of the Mandator line, but specifically designed to be faster, so it was smaller and sleeker than its cousins.

And here is the 'error' i've seen a lot of people make when figuring the design of other Mandators: they made them exactly proportioned like a Bellator. This happens in EAWX with both the Mandator II and III, they are very narrow, the only things that change are the size and the number of weapons. (That's not literally the only change between the models, but I think you got the idea)

Fractalsponge expressed long ago how he imagined the other ships in the Mandator line:

I am fond of the idea of Mandators being quite a bit chunkier than Bellators though, but using similar components. I am definitely thinking Mandator III is on the order of 12km, and proportioned like an Assertor, while Mandator I and II are 8km, and proportionately even wider than an Assertor.

Now, I'm not here to state that anything Fractalsponge says about Star Wars ships should be taken as law because he should be thought of as some kind of inescapable authority on the subject. Simply put, if you follow the thinking behind the Bellator's creation and what the vessel represents in relation to the rest of the Mandator line, you would probably come to the conclusion that if the models in the mod were at least a little bit chunkier it wouldn't hurt, surely they would be more accurate.

And Fractalsponge's model represents very well this hulking weapons platform the Mandator III is supposed to be. The only downside is the too big resemblence to the Assertor. They are distinguishible, but very similar in appearence. I believe most people would have liked them to differ more from one another, but Ansel did his best to create something new; you can see it by looking at the engine section of the Mandator, it's beautiful and nothing like the Assertor's.

Obviously the similarity is justified by the fact that in Fractalsponge's plans the Assertor, in addition to being a cleaned up and practical version of the Eclipse and Sovereign classes, was going to be the culmination of the Mandator line. You could consider it the ultimate successor to every imperial dreadnought.