mound, hillock
冢 centers on a raised earthen feature: a mound or small hill, often a burial mound. The hillock sense is a natural smaller-scale application of the same idea.
冢 combines 冖 (cover) with 豖, a pig-related component, likely suggesting a covered mound or burial site. The exact historical development is uncertain.
The top 冖 looks like a covering over the lower 豖, which resembles a pig. Picture a pig-shaped mound of earth covered with a cloth, like a small hillock used as a burial mound.
For チョウ, imagine a long-serving chief named Chou standing on top of the mound, surveying the land: Chou -> チョウ.