Well, you see, the Office of Naval Intelligence was trying to get a handle on you guys. Then they found this collection of ancient
sourcebooks tactical manuals, and suddenly it
all made sense!
Really, looking at the Imperium Navy from 40k it's obvious that the doctrine emphasizes survivability over firepower, though the sheer size and sophistication of the ships guarantees firepower too. I figure that someone put a LOT of thought into designing the ships with layered protection schemes, internal defensive cofferdamming, damage control, that sort of thing. Then they wrapped the whole thing around a 'merely' average main battery, and accepted that the ratio of defense to offense on their ships was such that it would take hours for it to beat down the guard of an opposing ship of its own class.
Which turned out to be a
really good idea, given that a lot of those ships have lasted for over ten thousand years. Which, given the Imperium's aversion to running away from a fight, the fact that those ships lasted for ten thousand years is pretty damned impressive.
In contrast, the Umerian design philosophy views defense as a necessary adjunct to getting the ship into position and putting power on target. So the ships
aren't designed for maximum survivability, not beyond the point required for them to do their jobs in the kind of action they're expected to go into. The dreadnoughts are fairly sturdy, but everything lighter tends to suffer from "eggshells armed with sledgehammers" syndrome.