How much rain reaches a cistern?
For a fictional catchment, 1 millimetre of rain on 1 square metre is 1 litre of water. This simple budget assumes an empty tank and one rain event. It is not a reconstruction of a Venetian cistern.
Hold rainfall steady and enlarge the catchment. Then lower capacity. Does a larger collection area still increase the amount retained?
Assumptions omit leakage, evaporation after collection, existing stored water, demand, timing, filtration rate, sand pore volume and water quality. Collection fraction is an illustrative input, not a measured historical efficiency. Water captured is not necessarily drinking water.