I believe the Defiant Class in opposition of the Borg is supposed to be a fleet of ships.
Imagine 15 Defiants in formation concentrating their firepower.
Resources to construct
1 Sovereign Class Ship = 15 Defiant lass Ships.
1 Sovereign Class Ship = 5 Defiant Class Ships.
1 Sovereign Class Ship = 1 Defiant ships.
The Defiant is a couple massive guns strapped to a speedy warp drive.
Because it's so much huger is it might actually be easier for the Borg to assimilate a Sovereign class vessel? But if it's only equally as easy to assimilate/lose a Defiant as a Sovereign, when it costs 15 times the resources to build a Sovereign it's wholly more economical to only take the little ships into battle against the Borg if any 5 little ships have the same fire power at barely a third the price in resources.