The Powercore 5000 is 5.7cuin, the Fusion 5000 is 9.4cuin. A 10W charger is 2.3cuin. 5.7+2.3=8.1. So the Fusion was 16% bigger than the sum of its parts, but when factoring the removed cable you're point is valid.
The Powercore 10000 is 7.4cu, say you used the same 2.3cu 10W charger (not quite fair) or the 4.9cu Powerport II 2 22W (more than the Fusion's 10000 15W) you get 12.3 cuin for 10000mAh + 22W charger. The Fusion 10000 is 14.3cuin for 15W charger, so the Fusion is 10000 is also 16% bigger than the sum of its parts but against a higher wattage.
So there seems more room for improvement for the Fusion 10000 than we could have got with the Fusion 5000. There is also something going odd with the 10000 Redux and 20000 Redux getting bigger.
This could just be Amazon sizings being wrong.