The dreadnaught originally may have been 2200 meters instead of 22,000m. This would explain the low mass. But weather it was a typo or not, the dreadnaught is now officially 22,000 meters as of False Colors. The ship does have an unusually low mass, though. The ship is not all bulk, but it is not really all hollow, either. Plus, the mass was mentioned as being off the scale--or something to that effect--in False Colors. Thus the mass could either be considered incorrect, or the dreadnaughts (as there are more than one) possibly are composed of very cutting edge and efficient materials. (very light weight and durable as opposed ot other ships.) Because now matter how hollow it is, there is no way a 22km ship is going to mass less than several 1km long carriers. The skeleton alone would weigh more. All ships in the WC universe are fairly lightweight. This implies that ConFed and the Kilrathi advanced in their material construction.
I'm not sure when the dreadnaughts were built. The only clue we have is in False Colors where they were describred a warship built by Thrakhath as part of his "final solution for mankind." This implies that they were possibly built aroud the same time the Hakagas. Or atleast they were part of the same design docs for next wave of warships. (Think F-22 and Commanche helicopters.)