Binary Large Objects (BLOBs) are stored in SQL Server as either text or image datatypes. The SQL Server maintains a pointer to the BLOB data. Thus, BLOB data is not stored on the data page along with the other fields in the row by the SQL Server. The 16-bit text pointers are used to store actual data in 2KB pages. So, about 1800 bytes is available for actual data storage if the column is not NULL. However, if the column is explicitly set to NULL, there is no need for the text pointers. The storage size will then be 0.
This means that storing data in BLOBs increases the storage requirements in 2KB increments. Besides storage size, there are also functional limitations with BLOBs. For example, if you use a WHERE clause to search on a text column, you are limited to using the LIKE operator.