Hi ,
InstallBuilder uses different methods to calculate disk space depending on the OS. For example for OSX we use the df -k command which outputs the value of the available disk space. Could you launch the installer, wait until the ready to install page and check the output of the df -k command in the terminal?
To get the output in MB you need to divide the result through 1024. Can you check how the resulting value compares with the available disk space?
Regards,
Michiel