Becoming twice as big generally means conquering. Establishing branch offices means you aren't conquering those planets. It doesn't scale as quadratically as you'd think.I don't have a problem with the concept, but the scaling. If a Megacorp becomes twice as big, with twice as many branch offices, that means their *base* diplo power will be about twice as much (ignoring power from tech for the moment, which is a complicated issue). But then they're also getting twice the *multiplier* from corporate embassies, so you end up with quadratic power growth. In practice, this isn't a problem on small galaxies but gets a bit crazy on bigger galaxies, as in the OP screenshot.
Instead, I would suggest that the corporate embassy should grant a chunk of base diplo power from economy based on the trade value of the planet. That way you can still get some extra diplomatic heft as a Megacorp, but you're not also getting a huge multiplier on every pop/ship you own and every resource you make inside your sovereign territory.
The idea of diplo power based on the trade value of the planet is interesting though. I strongly suspect that if it was a "flat" value, it would have to be quite significant to be worth building.