How to create more space for OS X in VirtualBox?
Try to allocate more disk space in your VirtualBox configuration (before booting the machine).

In VirtualBox: 


Click on your machine
Configuration
Storage tab (on the left)
Click on your disk
Click on the button in the top right (check fig. 1)
Create new disk (it seems that the data will be moved from the disk to another)


Fig. 1 :