I'm playing on an Insane scale map and there are literally hundreds/thousands of colonizable planets available to the seven civs vying for galactic domination. At mid-game -- I've only uncovered about 50% of the map -- I have 82 planets under my control. Of those, 53 have ZERO Approval. It doesn't matter that I'm getting a 300% boost from Approval Relic studies. It doesn't matter that the colonists have been taken from seriously overcrowded planets and can stretch out on Type 20+ planets. It doesn't matter that they have all of the Approval boosting structures built. It doesn't matter they are very well defended against whatever evils the galaxy may send their way. EVERYTHING positive in the way of Approval/Morale boosting in both points and percentages is offset by ONE negative modifier: Big empire penalty -16.
A truly galaxy-spanning empire WILL encompass hundreds of worlds. Sure, it become difficult for a central government to stay on top of a sprawling empire that spans hundreds/thousands of light-years. But inevitably, it WILL become possible. If pre-radio Earth empires could be maintained by communications that traveled no faster than a wind-borne ship, an advanced high-tech society WILL find a way to communicate faster than light. (They already do that in the game, simply by using FTL courier ships.) Governmental forms most likely will have a system of Viceroys and Planetary Governors reporting to the Home Office. It's really not necessary to have the central government micromanage EVERYTHING to keep the empire running smoothly.
It's quite arbitrary to make creating a HUMONGOUS empire impossible, based on ONE negative factor. Citizen Approval and Morale is easily improvable to the point that even distant citizens can take pride in being part of the "largest empire in the galaxy!" [Then again, if distant planets are mismanaged, the natives can become quite restive.] As long as "life in the empire" is good and pleasant, the number of planets in the empire really shouldn't matter.... all that much.