[DennisP]

I figure technological advancement is probably an S-curve, and by the time you have a galactic empire, things have leveled off quite a bit. There's probably not much change over a couple decades. (And in Star Wars, things actually seem to be decaying a bit.)

Also you seem to be assuming that the Death Star is big just for the sake of being big. More likely, most of the mass is the actual planet-destroying weapon, as well as the engines for moving it around.