Karachi’s population has only increased in size.
There is a constant influx of Pakistanis from villages and small towns. Heck Kemari is full of Bangladeshi folks. Sorabh Goth is probably bigger than Peshawar in terms of population. And then there is an equation of producing like rabbits.
The reason why Karachi’s population will be kept to a bare minimum is because if they don’t, more local bodies, provincial assemblies, national assemblies seats will be assigned to Karachi. This would mean Karachi voters will hold more sway and reduce PPP stronghold.