The overall performance in an I/O-bound application is determined by the number of disk drives on a system, not by the amount of space available. A single disk drive might not be able to deliver the number of I/Os per second that are needed for your Adaptive Server application.
To achieve your performance objectives for an application, you must have enough disk drives to give the necessary number of I/Os per second.
Your disk drive requirements may not be directly related to the size of your database. Depending on the amount of I/O you need, you may have free space on your disk drives.