extend, deep, large
覃 centers on spreading outward or reaching far: extending in space, depth, or scale. The meanings 'deep' and 'large' are natural extensions of that spreading, far-reaching idea.
覃 is thought to combine 覀 (a covered container) with 早 (early/sun), possibly suggesting something spreading out from a container like the morning sun. The exact historical development is uncertain.
覀 looks like a covered basket, and 早 suggests the sun rising early. Imagine the sun's rays extending out from under a basket lid, spreading deep and wide across the land.
For タン, picture a tan-colored basket (覀) spreading its contents out under the early morning sun (早): tan -> タン, and the basket extends its reach.