The storage capacity of any medium is fixed: conceptually, it has a certain number of "byte storage cells" and no more. Compression allows you to represent the original data in fewer bytes, and in a virtual sense "get more on a tape".
An LTO2 tape has a fixed "byte storage cell" capacity of 200 GB. You can't write 201 GB to it, but you can reduce the data by compression such that it takes up less space. Tape drives provide transparent compression in hardware to make that possible. Richard Sims
