Data compression is needed because it allows the data to be stored in an area without taking up an unnecessary amount of space. Data compression uses a series of algorithms to reduce the amount of real space that the data would normally take up.
The amount of data that is shown when it is compressed is dependent on how the data has been compressed. If there is an extremely large amount of data in a sequence, it will be compressed to a large size. If there is a small amount of data that is compressed, it will not take up as much real space as compressed large amounts of data.
Determining the amount of space that the data will take up is dependent on the algorithm that is used to compress the data. If the algorithm is used properly and formatted specifically to the data, the data can take up nearly no real space. If a generalized algorithm is used, the data may not fit into a compressed area as well as if a specific algorithm is used. The algorithm is the determining factor on how well the data will compress, how much space it will take up and how much of the data will be available after compression.