Is the value of the storage capacitor (VCAP) for nvSRAM proportional to its densities?
No, the storage capacitor (VCAP) value is not proportional to its densities. The capacitor value on the nvSRAM VCAP pin is determined by the VCAP current drawn during AutoStore (ICC4) and the voltage driven on the VCAP (VVCAP), as mentioned in the datasheet. The charge stored on the capacitor provides the store current during the AutoStore operation. nvSRAM devices that charge VCAP to a higher voltage require a smaller capacitor than nvSRAM devices that charge VCAP to a lower voltage.
See AN43593 for a discussion on the selection criteria for the storage capacitor (VCAP) for Cypress nvSRAMs.