So, after doing some fiddling, I must ask: why does the Vulpes use an XXL engine?
Because it is Davion, and they always have the most advanced tech. I mean, that is what they are known for.
Most of the rediscovered Star League Tech was deployed out of NAIS, for example. The Davions were, frankly,
one of the biggest sufferers of "New Toy Syndrome" in the Universe. Sure, Kurita slapped ER PPCs on everything,
but Davion would put ANY new tech on things, even if it did not make sense.