When major PR firms merge, they’re usually consolidating in order to stay on top. It’s a little different for former standalones Pmk-Bnc (which has merged at least four times in 40 years) and Rogers & Cowan: Both are owned by the publicly traded Interpublic Group (2018 revenue: $9.71 billion). Now, they will form a single firm (yet to be named) that will represent more than 500 clients and 30 brands.

So why the change? The combo creates a more impressive client list, and will allow them to trim the expenses and other redundancies that come when two entities merge. It’s the easiest and fastest way to become bigger while meeting the demands of their corporate overlords.

Of course, bigger is not always better. Still, when the new company officially launches January 1, it will dwarf Kelly Bush and Mara Buxbaum’s ID-pr. It will also become one of the top 20 PR firms in the world,...