First, we should take care to avoid over-generalization about mergers. Second, there are mergers and there are acquisitions. It is at one end of a continuum that includes affiliations, federations, and consortia.
Merger is at the end of the continuum where institutional autonomy is most compromised.
Fourth, economy of scale can be illusory in some mergers. Although there are a number of sound studies of optimal institutional size, few result in a specific figure. Most result in ranges. Nevertheless there is a surprising number of mergers between institutions that even together do not reach a size at which significant economies of scale can be realized. Note, for example, Barat's preference for DePaul over Lake Forest.