Minor detail: No it doesn't. Defiants are actually noted as being unusual for having the same carrier capacity as a Dictator. Though, also that some battlefleets 'do not know what to do with it'.
It's actually statted separate from the other Voss hulls in BFK.
In which document are Defiants said to have the carrier capacity of a Dictator? And is that document available online? I'm not trying to challenge it, I'm just ignorant of this particular factoid.
Regardless, it got me thinking: The number of launch bays a ship has represents not really the maximum attack craft capacity (something which can never really show up in the game) but more the speed with which those craft can be deployed - how many of its X number of squadrons the ship can deploy in the 30-60 minute time window represented by one game turn.
If the Defiant had 2 launch bays, and the special rule which doubled its attack craft control cap to 4, that could reasonably represent something with as high a capacity as a Dictator, but still not enough attack craft punch in one turn to upset anyone.
On second thoughts, the control cap still doesn't represent theoretical maximum 'capacity' well, only the number of squadrons the air traffic controllers on the ship can handle simultaneously, which is likely much less than the capacity. Maybe the maximum launch bay capacity is something which needs no real representation in the game, like the capacity of a Leman Russ tank to carry spare battle cannon shells.
Anyway, just my thoughts, for what they're worth...