The optimum minimum size of a bank (which is difficult to define anyway) has presumably increased in recent years, not least on account of the need to invest in state-of-the-art banking technology, and to ensure more efficient compliance with supervisory requirements.
Mergers and takeovers involving major, mainly private institutions must undoubtedly be seen against the background of the need to secure a better position on national markets. Ultimately, however, all mergers are motivated by the wish to cut costs by exploiting rationalisation potentials, for instance by reducing the branch network, some of which is then redundant.