The truth is that the product with the 16% reduced capacity (Switch 2 Pro controller) is 7g lighter and the one with the 5% increased capacity (Gamecube controller) is 5g heavier.
Besides those two, the general idea is that the capacity is the same with 2-3% extra weight.
The Switch 2 itself loses 1% of battery capacity, most other products none at all.
Your framing seems a bit selective to the point of being misleading.
The basic Joy-cons have no change to their battery capacity.
https://www.ifixit.com/Guide/Joy-Con+Battery+Replacement/113...