big
侊 centers on largeness or bigness, describing something of great size or stature.
侊 combines 亻 (person) with 光 (light), likely suggesting a person of great radiance or prominence, hence 'big'. The exact historical development is uncertain.
A person 亻 radiating light 光 appears larger than life, so 侊 means big.
For コウ, imagine a big person named Kou standing in the light: Kou -> コウ, and his size makes him stand out.