If you said shrinking to remove a layer of a government bureaucracy, I would agree with you. However, most large companies are not run as inefficiently as our government, or the companies do not stay in business for long.
If there is a big bank breakup, I believe most jobs would be shifted around to other companies. I do not believe there will be a shrinking of the bureaucracy. It will be more like flatting out the bureaucratic charts. Some mid-management may even receive a promotion when hired by a new start-up company, since the number of top level officers will increase.
However, a breakup also provides an opportunity to lower the wage scale and benefits for the employees moving to new companies. It also provides a good time for big companies to get out from under some of their expensive retirement plan obligations by removing employees nearing retirement age.