Your PlayStation 3 temporarily needs extra space on the HDD to install content after you have downloaded it. The space needed to install is roughly equal to the size of the content. For example, to download and install a 14GB game you need approximately 28GB of free space on the system HDD. Once the content is installed the extra space is freed up.
If you buy a game or video from PlayStation Store on your PlayStation 3 and you do not have enough free space, we'll let you know before you complete your purchase.
You can make space to download the content before or after purchase by deleting content or by using one of these methods:
PlayStation 3 systems use some HDD space to run the operating system and various apps. You can check the available space by going to [Settings] > [System Settings] > [System Information].