But you choose amount of RAM dependant on maximum memory usage. Therefore swap space (being proportional to RAM) becomes dependant on largest expected allocation also. It wouldn't be wise to build system with 2Gb of RAM and 4Gb of swap space, when you need 6Gb of memory at peaks: such a system would be slooow. It may be not wise to buy 8Gb of RAM when 5Gb is the maximum that might be needed.