Pakistan has always had four provinces, even though its population has increased exponentially. But the very idea of increasing the number of provinces makes Sindhi speakers abusive. There is a good reason for this. Sindh is the only province that is dependent for all its revenue solely on one city (Karachi), which provides almost hundred percent of its budget. Naturally, if most of that money is spent in Karachi (if the province is divided), people living in Sindh's other cities will have to pay all kinds of taxes (a big chunk of which will of course go into foreign bank accounts so the rulers can spend the money when they go for their annual vacations).
The arguments against having more provinces of course carry some weight. For one thing, the new provinces will require funds for paying legislators and their staff, which will require common citizens to pay more taxes, as feudal lords will exempt themselves from taxation. So, whether new provinces are created or not, the ordinary man will continue to suffer.
There is a way to solve the problem, and that is to give adequate funding to local government bodies, as envisaged in the Constitution. But again, for Sindh's rulers, this means less money for themselves and their supporters, so there's a slim chance of that happening.