And as been stated in dialogue that the Defiant carriers a big punch in a small body. Proving that bigger is not better.
Why would it prove that? The Defiant
is good for a small ship, but she's never torn big capital ships a new one, the way the larger hero ships have done. The damage she dishes out on Cardassians cruisers in her titular episode is rather superficial, and she never quite kills a capital ship in any other episode, either. Bigger does seem to be better in that respect.
Although bigger certainly ain't better for survivability
in those later battles. But that probably has got more to do with who's a hero and who's not.