GSMArena says both phones take 10W power input.

One likely candidate is the type of power regulation used for charging. A Li-ion battery is about 3.0V when empty, and 4.2V or 4.35V when full. Basic USB power is 5.0V, or higher for various fast-charge technologies neither phone uses. To provide the correct voltage during battery charging, a charging circuit could use an expensive, bulky, efficient switched mode power supply, or a cheap, compact, inefficient linear regulator. The latter just turns the voltage difference into heat.