It isn't. I went from Apple's 16 Pro to an Oppo Find N5. Battery size 16 Pro: 3,582mAh. Though in fairness the Oppo is the same size as an iPhone 16 Pro Max, which has 4,685mAh.
Battery size Oppo Find N5: 5600mAh 56% more than my old phone. 20% more than the 16 Pro Max. Silicon carbon batteries.
It's beautiful what can be done if we go with modern technology and not Apple's profit maximising regurgitation of the same same for many many years.