Microsoft SQL Server 2014 Unleashed

Row-Level Compression

Row-level compression isn’t true data compression. Instead, space savings are achieved by using a more efficient storage format for fixed-length data to use the minimum amount of space required. For example, the int data type uses 4 bytes of storage regardless of the value stored, even NULL. However, only a single byte is required to store a value of 100. Row-level compression allows fixed-length values to use only the amount of storage space required.

Row-level compression saves space and reduces I/O by

Reducing the amount of metadata required to store data rows

Storing fixed-length numeric data types as if they were variable-length ...