My suspension of disbelief was held aloft on the Defiant's size vrs capability issue simply by this:
It had no brig, no holodecks. Likely limited bare-bones replication, little or no recreation areas. Smallest of crew quarters, and simplest and tightest of accesways and so forth.
It was specifically designed to be as small and light as possible, which usually indicates expensive or even experimental materials, with limited supply and inordinate costs (think all-carbon-fiber car).
It was an engine, upon which had been strapped very large guns, a shield generator, and a bridge, some crew cabins in what space was left between and a thin hull of insanely expensive and rare material stretched over it all.
Posters with better knowledge of the episodes point out replaceable weapon power cells instead of the standard ability to never need replacement. Think in terms of batteries instead of power draw (allowing for a smaller core, as nothing is pulling form it but minimal systems, not even the guns).
This was not a ship meant for long journeys alone. This is an older attack submarine. Compact, efficient, deadly, and entirely dependent on other ships and facilities for long-term needs. In game-terms, I would expect enormous speed and handling, innate bonuses to DPS and accuracy, or Crit Dmg and Strength alternately, but rather limited weapons battery and VERY slow health regen, possibly even innate debuffs to BO healing abilities when applied to yourself (Emergency Power to Shields, etc).
All the other tier Escorts seem to be less overspecialized in design.