Space calculations determine the amount of free disk space in all mounted filesystems, the disk space required for each subsystem, the temporary requirement for disk space during the installation process, and the net change in disk space if a subsystem is installed or removed. If a detailed space check is requested, Inst also checks the disk for any deleted files, and for any local modifications to the installed software.
You see this message when Inst performs space checking:
Calculating sizes:The space checks performed during list and step processing calculate the net amount of space that your selections will require after installation. The space checks performed during go processing calculate the net space requirement and the additional space that will be required during the installation process.