Why were haniwa figures hollow?

Haniwa figures were made hollow primarily to withstand the stresses of the firing process. When creating large pottery sculptures, maintaining an even wall thickness is essential to prevent the clay from cracking or exploding in the kiln.1

These figures were crafted using techniques similar to those employed for hand-built pottery, such as coiling, pinching, and slabbing, which allowed the artisans to build the forms directly as hollow structures. By ensuring the walls were of a roughly even thickness, the potters could successfully fire these large, unglazed terra-cotta objects, which were then placed on and around the burial mounds (kofun) of the Japanese elite during the Tumulus period.21