First time seeing the Defiant

For how sma it was, it punched way above its class.

Like look at the Big D, if they separate the saucer section; it’s suddenly a fraction of its mass with the same firepower.

You can do a lot if you’re not using children as shields.

Worf is visibly upset at you calling his ship "small."

You could say it defied her expectations.

Doesn't need to be.

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.

It's not the size, it's the warp coil disturbance!!

There once was a little ship and it took a little trip!

Look at it this way: The Defiant had more of the ship built as a set than any other Star Trek Hero ship.