I have to agree. The current values don't make much sense (especially the weight and the cost). However, I'm certain, that they will still be tweaked.
The values in GalCiv 2 weren't perfect either (mostly the weight again). However, they were still more believable. Here are some examples:
Terran scout (tiny)
length: 36m
span: 49m
weight: 3.5 tons
Terran star fury (tiny)
length: 33m
span: 49m
weight: 11.4 tons
Terran defender (small)
length: 43m
span: 81m
weight: 18.5 tons
Terran colony ship (cargo)
length: 933m
span: 503m
weight: 1.5 million tons
Terran survey ship (medium)
length: 299m
span: 256m
weight: 0.001 million tons
Terran battle axe (medium)
length: 299m
span: 218m
weight: 844.2 tons
Terran destroyer (large)
length: 1133m
span: 271m
weight: 0.118 million tons
Terran dreadnaught (huge)
length: 1583m
span: 1464m
weight: 13.8 million tons