If not otherwise set on the command line, the sizes of the initial heap and maximum heap are calculated based on the size of the physical memory. If phys_mem is the size of the physical memory on the platform, the initial heap size will be set to phys_mem / DefaultInitialRAMFraction.
DefaultInitialRAMFraction is a command line option with a default value of 64. Similarly the maximum heap size will be set to phys_mem / DefaultMaxRAM.
DefaultMaxRAMFraction has a default value of 4.