As another point of comparison, a couple other weapons with small mount versions get roughly two of them shoved together in a medium mount.
IR Pulse Laser -> Pulse Laser costs twice the OP, gets twice the damage per shot at twice the flux and +100 range
Light Needler -> Heavy Needler costs one less than twice the OP, has twice the burst damage at the same flux/shot. Not quite twice the DPS because it takes longer between bursts, but given that it costs less than two light needlers, seems fair.
A lot of missiles follow this rough pattern too, though usually with an extra OP or two or a slower fire rate traded off for 3-4x the ammo instead of just double. Still roughly a trend of one mid slot is worth about two small slots.
Arbalest costs twice the OP for 67% more damage and a very marginal flux/damage bonus that's outweighted by the lower OP/DPS. Given no slot restrictions I'd always just take two light AC. And that's not even getting into the light dual AC.
Although the Arbalest has more damage per shot, all that does is make it a somewhat (but not even incredibly, because it starts worse) better weapon against armor - unless the target has over 566 armor, then it gets no advantage. And honestly, they're both pretty terrible against armor so the difference is marginal and only really worth much against smaller ships you can't hit with the arbalest.
Heavy autocannon also looks pretty bad when compared to two light autocannons too, unless you really want that 100 range it really just costs more OP to be quite a bit worse. Still think they could both use some tiny buff. Still think -1 OP for arbalest so it at least costs the same as the railgun and it's just a tradeoff of efficiency and damage/shot vs accuracy, instead of also costing more too.