Talked about this in another thread.
I'm basing the mass figures on the relative densities of real-world spacecraft and aircraft. Apart from the fact that making the ship as light as feasible reduces strain on the engines (and thus provides certain advantages for the lighter ship over the heavier one) there's also the fact that less-dense construction materials react more favorably to hyper-velocity impacts than denser ones, since the low-density materials will not transmit shockwaves nearly as well as denser ones. There's also the fact that the stronger materials used in starship construction means less internal space is required by structural support work, which means a lot more of that internal volume is truly empty space.
Basically: the Enterprise is a giant space shuttle, not a flying aircraft carrier.