In fact, according to Wikipedia, many of them appear to have masses similar to oceangoing ships of similar length. This gives reasonable results for ships that have a similarly long and thin shape, like the Phobos, which I estimate to have a density of .24 T/m^3 (a.k.a. g/cm^3).
But for wider ships, it's too low. For instance, the Xenophobe worldship is only 15% more massive than the Phobos, despite having nearly 4 times the volume by my estimate. And the Xenophobe ark is only 2.5 times as massive, despite having probably 15-30 times the volume. (It's a bit hard to judge how long the cylindrical section is and whether the bottom is rounded or flat.) The CSC is particularly hard to judge, but I think its mass is probably also low by around an order of magnitude.
I got to thinking about this after seeing this post showing off a ship that's 1000 m long and 9 million(!) tons. That seemed ridiculous compared to vanilla ships, but after estimating its volume I concluded it was reasonable.
Lengths taken from Human Space HD.