I had a look at the ships described in Rogue Trader and I found the cruisers suprisingly lightweight. I will use the Lunar cruiser as an example.

With a length of 5 km and a max beam of 0,8 km, I have approximated the volume of the ships as a 5x0.4x0.4 km 'brick' - sounds fair fo the rather blocky design. Which give it a whopping 800 millions cubic meters of volume. At 30 megatons, it means an average density of 0.0375 - it's not a soap bubble but that's coming awfully close for something that's heavily armored.

Using the 'Bismark' battleship (50 000 tons, 250m length, 40m width, 10m draft), a very rough approximation (250 x 40 x 20) gives a density of 0.5. Even halved, that's still ten times more.

Which means the Lunar cruiser should weight about ten time more, 300 megatons instead of 30.

Note : it would makes it more coherent with the lightweights : a raider or frigate is about 1/3 of the length, wich means a 1/27 of the volume. If we assume the light cruiser to match the hull shape of a raider and the cuiser that of a frigate, it would make the Dauntless 150 megatons and the Lunar 180 megatons. Which is more plausible, density-wise.

Another forum I visit has known about this issue since the ship preview.

Quoting from that thread to give you an idea just how lightweight the hulls are: Mass / (length*beam^2) for density, compare to real world substances and structures. For instance, if the Jericho-class, the first listed hull, was made entirely of styrofoam, it would mass about 18 megatonnes. The listed mass is 9 Mt.

Someone there did email FFG about the issue then, they posted the response in that thread.

I can admit having lightweight numbers floating around - the raider and frigates aren't too far off. But what really rubs me the wrong way is the huge variation in mass-to-length rate between the frigate/raiders/transport hull and the cruisers, wher tonnage is simply scaled with length while each and every picture gives them similar hull proportions.

It also means there's a huge gap in hull sizes with nothing in between a 1,5 km raider/frigate and a 4,5 to 5 km cruiser. Reducing the cruisers to 2,5 km for a light one and 3 for a standard would make sense in that regard.