I never liked the defiant either, it is not very credible. It is a tiny ship and it seems to outgun most warships in the quadrant. Destroying jem hadard attack ships and cardassian galors is like turkey shooting to it.
And it's not very consistently displayed. In some shots it appears next to a galor and it's tiny compared to it (especially in the ep where tom riker stole it), in other scenes it looks bigger.
Worst, the okudagram clearly shows 4 decks only, and they sometimes send the turbolift to deck five!