Look at the SE versus the 5s. Same battery size (practically), same screen size, but the SE has almost double the battery life of the 5s.
What? Citation, please.
iPhone 5S:
Non-removable Li-Po 1560 mAh battery (5.92 Wh)
Stand-by Up to 250 h (2G) / Up to 250 h (3G)
Talk time Up to 10 h (2G) / Up to 10 h (3G)
Music play Up to 40 h
iPhone SE:
Non-removable Li-Po 1624 mAh battery (6.21 Wh)
Stand-by Up to 240 h (2G) / Up to 240 h (3G)
Talk time Up to 14 h (3G)
Music play Up to 50 h