Much as in real life, the distinction between an escort or destroyer and a light cruiser might be as much about what the ship is intended to do as it is doctrinal?
In real life we have the
Ticonderoga class cruisers and
Arleigh-Burke class destroyers at similar sizes, as well as the
Slava class guided missile cruiser being the same size as many modern destroyers. (The distinction is a bit murky here because there are so few modern cruiser classes which vary substantially in loadout, and the role heavily overlaps with a destroyer in practice.) Frigates can also be very similar in sizes to some destroyers, but are generally understood to have less of an air-defence and more of an antisubmarine role, as well as being cheaper. Or there's the somewhat ill-fated Littoral Combat Ship, which is similar in size to many modern frigates but was intended for a different set of roles.
To me, the
Sabre-class being not that much larger than the
Defiant class* but classed as a "light cruiser" rather than an escort would suggest to me that it intended to do more cruiser-like functions, I.E. to be more multirole, able to cruise around and respond to lots of different contingencies. This might imply more cargo and shuttle capacity for ferrying personnel and supplies, perhaps greater endurance or speed, and even more scientific facilities. This is still Starfleet and ships must still be expected to perform peacetime duties, and reconnaissance is important in warfare, not to mention the potential value of science labs for devising on-the-fly countermeasures when fighting the Borg. Essentially a modernised version of the
Miranda.
The
Defiant class, on the other hand, is expected to do one thing well, and that is go toe-to-toe with ships several times its mass and give a good accounting of itself. The "escort" in the name might also suggest an additional and more specialised role of being able to bodyguard more omni-capable flagships like the
Sovereign and
Galaxy classes. These flagships can add huge value to a task group with their extremely powerful sensors and extensive medical facilities, transporters, stories and shuttlebays etc., but might be in danger of getting jumped by more numerous smaller adversaries.
*(Google suggests the Defiant actually outmasses the Sabre at 355,000 tonnes to 310,000 tonnes, although whether this is accurate I don't know.)
Some sucker at some point made me a top-level clan member in the SB fleet and I've never let go of power, baby. Nevermind I haven't played properly since the Iconian War, which was eight years ago. Except to grind out some new reputations and so on, of course. My cruiser is running mark 15 gold equipment so I must have had a binge somewhere in the last half-decade. After a certain point STO just turns into e-peen measuring over who can kill the Borg quickest in The Conduit (Space) anyway.
I for one welcome our new (old)fleet overlords. Also man, the Iconian War plotline, I remember playing when that was still coming out. Good times!
Would it also be alright to get an answer question I asked you in the post? Or is the idea that we should be tormented by uncertainty.😜