What size is the backup? Remember, when you backup a database - you are not just backing up the data. You are backing up any indexes, reserved space, unallocated space, etc.
For example, you have a database with 1.4gb of data. But you've had lots of transactions and there's some unallocated/free space of 10gb. When you backup the database you are actually making a 11.4gb backup and that's how much space is needed for the restore.
Look at your databases in Enterprise Manager. Right click on the database, select View and click on Taskpad. Look at the graph of the database size. That will show you ACTUAL size broken into data and unallocated sizes.
-SQLBill