You've added a new column, but SQL Server - if the default value is a runtime constant - will not update the existing pages that hold the table's data, it will be only a metadata change and that's the reason you see no increase in the space used.
For all intents and purposes (as Aaron mentioned in a comment), the values are there - and a
SELECT will reveal them.
Think of it as SQL-Server black magic. It manages to store information in 0 bytes. Remus Rusanu has a nice blog post about this optimization and more details about what (magic) happens in the background and when you subsequently update the column:
Online non-NULL with values column add in SQL Server 2012
The basic info about it, in MSDN docs,
NOT NULL Columns as an Online Operation
Starting with SQL Server 2012 Enterprise Edition, adding a
NOT NULL column with a default value is an online operation when the default value is a runtime constant. This means that the operation is completed almost instantaneously regardless of the number of rows in the table. This is because the existing rows in the table are not updated during the operation; instead, the default value is stored only in the metadata of the table and the value is looked up as needed in queries that access these rows. This behavior is automatic; no additional syntax is required to implement the online operation beyond the
ADD column syntax. A runtime constant is an expression that produces the same value at runtime for each row in the table regardless of its determinism. For example, the constant expression
'My temporary data', or the system function
GETUTCDATETIME() are runtime constants. In contrast, the functions
NEWSEQUENTIALID() are not runtime constants because a unique value is produced for each row in the table.