One of the reasons that warships are "big" is because they require lots of "stuff". This comes in two forms, stuff for the peeps and stuff that goes boom. The former is taken care of via the replicator so you no longer need storerooms for food, spare parts, etc. The second requirement is covered because the only expendable armaments are photon/quantum torpedoes which are a fraction of the size of actual torpedoes and missiles. Everything else is phasers, which just requires energy cells.
