If you move (or migrate) a FileVault-protected account (home directory) to a Mac that has more hard disk capacity, the space available within the protected account will be less than the actual space available on the hard disk. FileVault is a feature of Mac OS X 10.3 or later.
To make more space available for the protected account, follow these steps:
Tip: Back up the contents of your Home before starting the process—if security is a concern, create an encrypted disk image on your destination volume and copy files to it. You can choose Home from the Finder's Go menu to see all the stuff in your Home.
As Security preferences states, turning FileVault off and back on may take a while, depending on the size of your Home.
Tip: If you do not have sufficient free space on your hard disk, you will need to temporarily reduce the size of your home directory, for example by copying a large amount of data in your home directory to an external hard disk, then deleting the stuff that you moved from your Home. After copying, log out or restart so that FileVault can reclaim the unused space.
When FileVault is enabled, the stuff in your Home, such as desktop contents, documents, and account preferences, are transferred to an AES-128 bit encrypted sparse disk image. A sparse disk image is one that grows in size as you copy more data to it. "Traditional" disk images (non-sparse images) are a set size no matter how much stuff is or isn't on them. In other words, a traditional disk image of 2 GB will always be 2 GB in size even with no data in it—a sparse image will be only as large as the data contained in it.
However, the maximum size of a sparse image is established when the image is created. Of course, the maximum image size can't exceed your available hard disk space, so if you migrate the account from a Mac with a smaller-sized hard disk, the FileVault sparse disk image's size will always be limited by the size of your former Mac's available space, unless you follow the above steps to create a new FileVault image.