Given how old these ships are, how many millions of kilometers of wiring there would be even in a Sword, and how infested with Vermin (that really do like to chew on cables for no reason except extreme risk of death - rats are seriously weird) they are, it probably takes a good few thousand working ceaselessly just to keep the thing operational. It's a full time job for 200 people just to keep the Forth Bridge painted, and the surface area on the humid interior of even a sword is going to be much vaster than that, let alone the exterior and fixing minor battle damage (such as a 50m wide crater blown in the superstructure - not worth going back to port over) is going to be an enormous workforce.
Then there's the standard 40k Trope of Insanely High Tech right next to the pre-industrial. It may be that the ship has Thrusters that can bring its entire bulk onto a new course in a matter of minutes, but to move just one into facing the right direction for a manoeuvre takes 500 strong men ten minutes on a treadmill.
It may be that the plasma reactors at the heart of a starship can put out enough power to light modern day earth, but just keeping it supplied with fuel requires 1000 men full time on hand pumps.
Now I'm not saying they were originally designed that way, but as the ships ages, certain pieces of technology fail that no-one knows how to replace and work arounds have to be employed. The manual labour EASILY mounts up.
... I'm trying to picture how hand pumps would ever work on a tokamak and not getting any hits...
Second: why would the interior areas of anything but certain sectors be humid? Excess water in the air would be extracted at life support for additional drinking water.
Granted, DCT and maintenance would be a few thousand men easily. The problem is that, unlike modern day earth, outer space tends to preserve equipment rather then destroy it.
It seems like the assumption people are making is the majority of the ship is hollow, as it would be in a modern ocean going ship. In space, the majority of it would probably be metal, the mass of it serving to shield the crew from solar radiation. This would, of course, be riddled with maintenance accesses, plasma conduits, gun ports, docking bays, electrical conduits, etc.
At one point I seem to recall having done the math that a Lunar, if it followed the same ratio as a modern ship, would only have something like 1/100th of it's interior pressurized.
Edit: Found it.
"A lunar class is approx 5,000,000,000m^3 (approximation based off of GW mini)
Approx 1/100 of that space will be pressurized areas accessible to the crew, or 50,000,000 m^3 (the rest being armor, engines, mechanical systems, exterior defense turrets, etc.) (used modern space craft for this estimate)
Three quarters of that would be given over to areas for engine access/fuel storage and weapon related areas (magazines, interior areas of macrocannon batteries, engine access corridors, etc) (used a battleship and GW mini for estimate), leaving 12,500,000m^3
Now, hanger bays, work shops and fabrication areas, medical facilities, kitchens, officers quarters, armories, escape pods, command and control, spare part stowage, cargo areas, and ships gravity plating. 3,000,000m^3 remaining. (used battleship for estimate)
Now we add berths for the men. Hmm... if we assume voidsman's quarters are the average for the remaining crew, and a voidsman gets a 6'x8'x10' berth (rather then a navy bunk) that's 3,000,000m^3 there for 20k men. We're out of space before we even got to life support and food storage.If we halve it (10k) everything fits, and there is even a little space left over. Which fits the Chambers model."
Admittedly, it's flawed, as it's based on the mini, and we all know how GW loves artistic license even on basic things like human bodies...
Weapon stats in BFG are not exact. When one says WB Firepower, the value is just representative of how strong it is. It is very much different from 40k weaponry. You can't even directly convert the ranges much less the strength between both game systems. RT which might come close still has a different game mechanic and so still can't be directly converted.
Achem: D'Art, in Rogue Trader, if you put a Macrocannon broadside (str 6) and a Titanforge Lance Broadside (str 2) on a Lunar class hull... it has Str 6 WB and Str 2 lance. Does this sound familer?
The weapons ranges don't convert directly, but the weapons str are a direct conversion. I won the contest over at FFG for doing conversions between the systems (admittedly, it was for the mimic engine, but...) I think I know how to convert between the two at this point...