For TOS and movie era ships, since nothing in dialog has EVER positively identified that big dish as a "deflector" anything, I happily conclude that the dish on the Enterprise is actually a ramscoop. It generates a huge cone-shaped forcefield in front of the ship that scoops up interstellar hydrogen which is then reacted with antimatter to power the warp drive. Enterprise specifically has big feature because it's designed to travel farther and faster than any other vessel.
Klingon ships have this too, built into the "wings" of their engineering hulls, and the Reliant has those vents on the underside of its hull. The smaller size on both ships indicates they probably can't refuel off interstellar hydrogen and have to "skim" from the atmospheres of suitable gas giants along the way. Both of those designs would be at a severe disadvantage traveling in uncharted space where you can never be sure that a suitable gas giant is available, but in charted regions where you know (more or less) where you can go to refuel, they'd be perfectly adequate.
I don't really extrapolate this into TNG+ because I frankly believe treknology decays rapidly into gibberish shortly after "The Arsenal of Freedom."