Its all about the gain of the illumination lobe. With this idea I'm widely speculating, well beyond the other things I said (I'm a mechanics guy not electronics).
The logic is this: Iranians sit down, look at the S-300/-400 and figure out that trying to achieve the same brute force illumination power levels requires a similarly large (=expensive) system and while Russians have 50 years of experience and are masters at this PESA technology, Iran has none. A race that can hardly be won.
Conclusion would be to try to develop a higher gain radar that achieves the same but at much lower power levels.
How this was managed is the big unknown, apparently not by some silly approach like expensive high power GaNs.
The auxiliary antennas could be just maintenance hatches, or SLC antennas that just improves ECCM capability and makes locating more difficult. But they could be responsible how higher gain levels were achieved.
Other details: The radar aperture of both, engagement and acquisition radar are use to create two separate, redundant missile up link channels to receive missile seeker data. So not only two different data-links are available, they also operate at a different frequency as the radar functions. A complete nightmare if you want to jam the system, or just apply self defense jamming to survive. The PESA of the S-400 does not offer this capability.
Its these details that makes these systems: S-300PM, S-400, HQ-9, Bavar-373 so feared; their kill-chain is so redundant and powerful that only kinematic evasion becomes a option for survival. All single sensor systems, from Arrow-2/3 to Aster lack this legality. An ARH seeker equipped SAM can be defended against via a self defense towed barrage jammer, namely the closer the seeker get to the target, the higher jamming intensity becomes... SAGG guidance completely negates this effect and dual-band SAGG... multiplies it... That's why all the fuzz about S-400, it's close to mythical. Once you enter its no-escape zone, next to nothing will keep you alive.
That's also why exo-atmospheric ABM systems or even Arrow-2 are more advanced(expensive) missile wise but overall system-wise the systems S-300PM, S-400, HQ-9, Bavar-373 and to some extend Aegis (less cost-efficient, high-tech solutions) but are the real masterpieces.
System cost is of greatest importance: Aegis with SM-6 is a expensive solution but it seems Bavar-373 costs less than half that of a export S-300PMU-2 battery and 1/4 that of export S-400. Internal Russian prices should be closer to each other, while the Bavar-373 applies usually more expensive AESA technology for its radars.
Irans requirement to establish full border coverage are at least 20 Bavar-373 batteries.
PS: I still remember 10 years ago when S-300 were not delivered... I was hoping for a LRSAM system that would be made of a AIM-54 Phoenix copy with a large booster attached to it. A TPS-43, JY-14 or Nebo-SVU (foreign radars), would detect the target, and the Phoenix SAM shot towards it, receiving updates via a ground data-link antenna.
With the technology Iran had at hand back then, this appeared as the most promising solution to create a LRSAM...
Bavar-373 of 2019 is magnitudes more potent and cost effective than that desperate solution would have been and compared to Chinese and North Korean copies of the S-300PM and P series, it is a indigenous system with Iranian signature.
