The distance between the metal bands holding the cylindrical structure together decreases from top to bottom because the pressure the water exerts increases with depth. The top band only needs to fight against the water at the very top of the tower but the bottom bands have to hold the entire volume from bursting out.
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“the bottom bands have to hold the entire volume from bursting out.”
Depth. Volume is first order irrelevant. Curvature of the walls vs. volume does develop at large volumes.