Population has a double effect on morale-a malus and a multiplier. 20B is -66%, while 25B is -90% (for DA/TA; for DL 25B is only -80%).
This means if you have 5 VRCs on the planet in question, and a 120% racial morale bonus (for instance), you'll get 5x40=200x0.33=+66 morale out of the VRCs to balance out your -66 from pop at 20B, and 120x0.33=+40 from racial morale. But at 25B, you'll only get 200x0.1=20 and 120x0.1=12, for a total of +32, combined with your -90 gives you a net -58, without even counting taxation. -58 to +40 is almost a 100 point swing in approval, so, as you can see, it's not worth it to have pop that high.
120 is quite low, actually, but it's probably about right for your map, and secondly, in DA/TA, morale is "capped" at 100% from racial-but only AFTER depreciation by population. What this means for you is the maximum racial morale that is useful in the two above scenarios is 295 and 1000 respectively (that's your hard cap, so to speak).
For the most part, the highest pop I've seen used is 22B, as that's an 8B TA colony + 2x7B adv farms. This gives you a malus/multiplier of -75%, which means your racial morale stops having an effect at 400, and in our above example, you'd get +50 from VRCs, -75 from pop, and +30 from racial for a net of +5. But that's atypical for population numbers this high; normally racial morale is maxed out or close to it-something on the order of 90% of its total value-and so all the buildings have to deal with is the tax rate. (Morale starts at 100, so we get -75 from pop, but +90 or +100 from racial bonuses means a net gain for us.) But this only really works because a VRC is still giving us +10 each (40*0.25)-it's much, much harder at 25B+.
Additionally, the bonus a planet gets for being greater than class 10 (class 11 and up) is a flat approval bonus; not a morale bonus. It gets added in at the end (+10). I could have sworn that the bonus for going Neutral ethically was also a flat approval bonus, but having checked all three of my versions the other day, I have found it is no longer, at least; it is now morale and subject to depreciation.