Are there any size comparison pics floating around of an imperial bomber and a manta? I'd be interested to see the AI difference.
My perspective is thus. Firstly, many races, such as Imperials, use completely different types of AC in space, for obvious reasons. The space versions are larger. The Fury interceptor, for instance, measures in its smallest variant at around 60 meters in length. I wasn't able to find the exact size of the Starhawk, besides 'larger', but we can assume rough fighter/bomber size differences.
Irrelevantly, it is also why I don't like the size excuse for why Marines get half the AC, as Thunderhawks seem about Fury size. Rarity to manufacture, limited amount of marines to pilot them makes more sense. And Mantas in a squadron and its all fine. Only thing that bugs me currently is the size excuse for both things. Its true they are shielded, so that and excellent weaponry are reason enough for the 4+ save.
As for Mantas as mini-escorts, they are far, far too small. A Cobra is the size of multiple aircraft carriers, and would cram into 10 or more tabletops if it 'landed'.