Why does SQL Server add a 4 byte integer to non-unique clustered indexes

Question

It is possible to define non-unique columns as clustered as well as non-clustered indexes. However, SQL Server adds a 4 byte integer to the indexed columns in case of a clus

Answer 1

A non-clustered index already includes the clustered index column so it can reference the exact row that it correlates to. Hence with the uniquifier on the clustered index, the non-clustered index would also include the uniquifier.

A good explanation here: Understanding and Examining the Uniquifier in SQL Server

Answer 2

I believe this has to do with the row locator.

Width of the nonclustered index row = Width of the nonclustered index column + Width of the clustered index column = size of column data type + size of column data type.