small kidney bean
豇 centers on a specific type of small bean, the cowpea or small kidney bean. Its meaning is narrow and concrete, referring directly to this legume.
豇 combines 豆 (bean) with 工, which likely serves as a phonetic component. The character was created to name a specific type of bean.
The left side 豆 means bean, and the right side 工 looks like a small measuring tool. Picture a tiny bean being measured out: that's the small kidney bean 豇.
For コウ, imagine a cow (コウ) eating small kidney beans from a trough: cow -> コウ, and the beans are 豇.