LTO-7 tape is 6 Terabytes, but it achieves 15 Terabytes with compression. That means the LTO compression algorithm is completely ‘lossless’ and it will compress data by default only if the data can be successfully compressed.

It doesn't mean all types of data are same to tape. Depending on the type of data, compression can help to drive capacity. Video and music files do not compress due to the random nature of the data. But for other types of data, you can take advantage of the LTO technology’s 2:1 compression capabilities (2.5:1 compression for LTO-6 and LTO-7 drives).

So when considering tape capacity, it's necessary to think about the type of data of your application, then make a plan. In the other words, you need to consider the many different ways to look at storage capacity.
Advancements in data compression also are part of the picture. On the other hand, cost is the other fact when you plan to use tape as your storage solution.
That’s a big advantage when you compare LTO tape with other technologies.  SSD typically provides less than 1 Terabyte of native capacity. That’s expensive when you consider cost per gigabyte. Hard disk drives today can offer up to 8 Terabytes of native storage, but it’s still more expensive when you consider
cost and the need to raid your systems for data protection purposes.

 

Ref:

http://www.lto.org/wp-content/uploads/2014/06/LTO16_0026_ValueProp_Capacity_01_2016_FINAL.pdf