Calling it a cap is misleading-at most it's a soft cap, not a hard cap.  Secondly, the number tends to be closer to 20B than 18B, beyond which point it just isn't worthwhile.

The simplest solution for your desire for massive planets would be to drastically increase the morale bonuses that structures give-think anywhere from 5 to 10 times as much as they are now.

Since there is a cap on how much population depreciates your base morale, as well as the morale bonuses on a planet (at 25B and above, you still get 10% of the original value), having virtual reality centers that give +400% instead of +40% will have them give +40% instead of +4% on a 25B+ planet, which should then be able to be stabilized with three to five of them, depending on tax rate.

You may actually want to create a new structure for this purpose, unlocked after virtual reality centers or an equivalent, that is simply a clone with higher stats.

But yes, either way, you're looking at editing XML (which isn't as hard as it sounds).

As an aside-you are aware that these planets are not significantly increasing your income, correct?  While higher population planets do help with the tourism income, as they generate more influence, your primary source of income is nearly always taxation, which is determined by the tax rate (multiplied by all applicable bonuses of course) times the square root of your population.  What this means in simple terms is that to double the income of a 25B population planet, you need 100B pop on it.  It also means that your 25B planet is bringing in slightly more than twice what your 6B initial colonies (DL/DA; TA is 8B) are.

First, I agree and retract my mis-use of "cap" to describe the equation that figures approval based on population size as it doesn't describe the situation well. Second, I understand the economy, and realize that the larger population can be a burden in that sense.  I'm just looking for a happy medium that balances being able to utilize 300% food bonuses without dropping my approval below 41%. So far shrinking output of the farms works but feels kind of like a weak solution.  BTW I tend to rely mainly on tourism and trade to fund my empire (at least later in the game when this food issue really becomes a problem as 300%+Intensive Farming = Not Good)

At any rate, using extremely high morale structures was something I hadn't considered, but seems unfair in that I could put one uber rec facility on a planet that has a low population and keep them happy even if I tax them to death.  I'll mull this over though.  Thank you to everyone who responded to my post.