The current 4–GB default configuration includes memory for the OpenSolaris xVM hypervisor control functions and 10 or fewer guests.
When dom0 is booted, the OpenSolaris xVM hypervisor reserves a small amount of memory for itself and assigns the remainder to dom0. When a domU is created, the Sun xVM hypervisor takes memory from dom0 and assigns it to the new domU.
There is no default configuration or a default amount of memory to assign to dom0, unless the user sets a dom0_mem value in the GRUB menu when booting dom0. The values that can be set are initial memory, minimum memory, and maximum memory.
If you are running known memory hungry apps in dom0, then you need more memory. Examples include cacao processes and Firefox.