A dton is a measure of volume and not mass.
1 dton of lead masses a lot more than 1 dton of liquid hydrogen.
I much prefer using mass to determine a vehicle's performance as volume rarely has any affect except when working out aerodynamics or target size mods, for example.
The tank's high mass is due, no doubt, to its armor. Because of the rules focus on volume, I don't think anyone looks too carefully at just how heavy BSD armor can be. A pocketknife made out of it would mass more than a broadsword.
The type 'S', on the other hand is 100dtons which is probably closer to massing 1100 tonnes, if you assume a starship is heavy but can still float in water. 1 dton of water masses 14 tonnes ( if you go by 1dton=14 cubic meters )
Some rules assume 1 dton=13.5 cubis meters as the standard. This is really close to 2, 1.5x1.5 meter squares on a deckplan with 3 meter separation between decks. The difference of 1/2 a cubic meter is unimportant as deckplans have plenty of leeway compared to design stats.
imho, basing things on mass make for a more 'realistic' ;P game