You haven't given enough information to answer the question.
It's not relevant how much population capacity a planet has, only how much it population it has that turn.
Based on your numbers, I take it you have 5.58B population if Imperial, 4.61B population if Interstellar Republic, 3.87B population if Star Democracy, or 2.847B population if Star Federation.
It looks like the AI has gotten an economic boom (double taxation income, doesn't show up anywhere), because otherwise the numbers don't make sense, as I've never seen the AI with a population high enough for those numbers, nor would such morale be possible then.
But the short answer is economic boom and population.
Taxation is 20.1194 * sqrt(population in billions) * (1+(racial bonuses/100)) * (1+(planet bonus/100)) * if(econboom=true,2,1) * taxratevalue/63 * if(starfederation=true,1.4,if(stardemocracy=true,1.2,if(interstellarrepublic=true,1.1,1)))
I don't recall how many digits beyond the decimal it's accurate to, but given the first term I'd suspect 4 to be a safe guess.
Taxratevalue is equal to the following, and adheres to different formulas for different taxrate ranges:
Taxrate Taxratevalue
0 0
1 1
2 1
3 2
4 3
5 4
6 5
7 5
8 6
9 7
10 7
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13 10
14 10
15 11
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35 24
36 25
37 25
38 26
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41 28
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48 32
49 33
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79 51
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85 54
86 55
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96 60
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98 61
99 62
100 63