1W A-size passive directional sonobuoys packing under 10kg while NATO standard G-size is under 6kg., but I think the sonobuoys dispenser pod for UAVs and the sonobuoy monitoring and control system (SMCS) are the main weights here. The weight of the SDS pod using in the MQ9B is around 130 kg. A system consisting of 8-10 sonobuoys may barely fit. When we talk about detecting subs, a MAD system may also be included. The AN/ASQ-508 Advanced Integrated Magnetic Anomaly Detection System (AIMS) package weight is 27kg in official sources. In the more compact 30W MAD-XR developed for helis, they reduced the weight to under 2kg. But, If we say that CATS is 55kg and a SAR/ISAR optimized for TB-3 is 30kg on average, AIS, conformal antennas etc, unfortunately I couldn't figure it out how to take off.
I guess distributed systems are the best for this class. An ASW+ISTAR teaming system consisting of 3-4 UCAVs can be designed. But in that case, it would be quite challenging to work in different sectors simultaneously.