IIRC, isn't the Drost IIa from after the jump-booms were in use? the design may have gained a few hundred tons in the process of being updated to use the new system.
a Drost I, or a Drost II ("no bloody a, b, c, or d" ;) ) might have been designed under 5K mass without sacrificing much, in order to allow it's use in dropshuttle bays.