GarageBand '11: Not enough space to complete the download after purchasing from Mac App Store
The first time you open GarageBand '11 after downloading it from the Mac App Store, you may receive an alert that there is not enough space to complete the download.
In order to use GarageBand, some additional content is required. If you do not already have the content installed, it must be downloaded. This requires an additional 1.2 GB of disk space.
Make more disk space available by moving some of your files to a different disk (such as an external hard disk or a USB flash drive), or by deleting files you don't need and then emptying the Trash.