Data_compression u003d row

The overhead associated with row compression adds a few bytes of space, but compression doesn’t save enough for there to be net savings.
In most cases, you’ll find that row compression is able to save some space–but not always.
In some cases, applying row compression will make your index bigger.

You can configure row and page compression on the following database objects:

1. A whole table that is stored as a heap

It stores fixed character strings by using variable-length format by not storing the blank characters.
NULL and 0 values across all data types are optimized and take no bytes.
The following table describes how row compression affects the existing types in SQL Server and Azure SQL Database.

Enabling compression only changes the physical storage format of the data that is associated with a data type but not its syntax or semantics.
Application changes aren't required when one or more tables are enabled for compression.
The new record storage format has the following main changes:.

The simple explanation is that it take fixed-length columns and makes them variable length, adding additional bytes for the overhead of tracking the changes being made

The link provided has a table that references the savings for the datatypes used