earthenware, earthenware vessel
坵 centers on clay-based containers: earthenware and earthenware vessels. The idea is shaped earth used for holding or storing things.
坵 combines 土 (earth, clay) with 丘 (hill, mound), suggesting a mound-shaped clay object or earthenware vessel. The exact historical development is uncertain, but the component relationship is straightforward.
Picture a mound of clay (土) shaped like a small hill (丘) and fired into an earthenware vessel. The left side is the material, and the right side suggests the rounded shape.
For キュウ, imagine a cute little earthenware cup: cute -> キュウ, and the cup is made of clay shaped like a hill.