Big cars with a seating capacity of 6/8/10/12 people and only driver is there or in some cases another companion is also there.

They take up a lot of space on city roads leading to congestion.

After all traffic congestion is a problem of too many vehicles for a limited stretch of road.

Why not demarcate city areas into full seating zone where only cars full to the capacity only are allowed? Or else you may charge a penalty for any car seen in full seating zone without full capacity.