broad, wide, spacious
廣 centers on physical spaciousness: broad, wide, or roomy. Its meanings all describe an area that is not narrow or confined.
廣 combines 广 (building) with 黃, which likely contributed the sound. The idea of a building with wide, spacious rooms is suggested.
The outer 广 is a building, and inside is 黃 (yellow). Imagine a vast yellow hall inside a building, so broad and wide that it feels endless.
For コウ, picture a wide, open court inside the building: court -> コウ, and the space is broad and spacious.