So I tested out a Civ 4-style maintenance system last night. Modded in a basic cost of 3credits for each colony hub; removed the maintenance of factories, research labs and econ buildings (in Civ 4, no building cost maintenance, only cities), and then changed LEP from -0.2 approval per colony to (-0.2 credits per colony) per colony.
This seemed to help a lot (though I may have set the stacking penalty too low). By the time I hit 20 planets, I had to stop colonizing because I was spending 140 credits per turn on colony maintenance (how much colony maintenance were you paying at 20 worlds, Marigoldran? Oh yes, I remember - zero). New colonies needed to be set at 50% econ to break even. Once i'd built up my core worlds a bit, and started showing some profit from my econ planets, I was able to start colonizing again. Now, in about turn 300 or so, I've hit 120 worlds - and setting up a new planet is costing me 27 credits per turn, so it needs to be REALLY built up before it can begin to pay for itself. About 50% of my planets are Econ worlds, and yet my budget surplus regularly sinks to zero during colony waves, forcing me to stop again.
The effect is pretty simple - there's no hard-cap on expansion, but every new world you colonize forces you to increase the % of your total worlds which are dedicated to economy. Small empires can get away with maybe 1 in 10 bank worlds, and so have 90% of their planets doing useful things - research and industry. Bigger empires find themselves lugging around an increasing dead-weight of planets which only exist to pay the increasing maintenance burden.
This counteracts the advantages of extra build queues and extra population growth, since so many of my new planets are just pushing everything into generating cash. I need to take an extra planet just to set on cash generation if I want to add one for industry or research. Eventually, I'll reach a point where I need to set every new planet I get to cash just to support itself, at which stage there's no longer any advantage to expansion aside from denying the enemy a planet.
The current LEP system doesn't counter the bonus of gaining build queues and pop growth effectively at all, since even with the penalties from being at 0% approval, you're still getting something from the planet. You're still better off having it than not. With the modded version, that changed completely - setting up an extra planet genuinely hurts until it can be brought on-stream. Class 5 worlds need significant terraforming and building up to make them worthwhile additions to the empire, as opposed to being worth it regardless.