extensive, wide, broad, empty
昿 centers on vast, open space: something extensive, wide, or broad, and by extension empty or vacant.
昿 is a rare character combining 日 (sun) and 広 (wide). It likely expresses the idea of sunlight spreading widely, but its exact historical development is uncertain.
The sun 日 spreads wide 広, filling the sky with extensive, broad light. When the light is gone, the space feels empty.
For コウ, imagine a wide, empty field under the sun where a cow (コウ) grazes alone: cow -> コウ, and the scene feels extensive and open.