Alter existing column to SPARSE results in more space used by the table?

Hi All,

Just learning about sparse columns now. I have a table of 300 million rows where 2 of the columns (1 INT and 1 NUMERIC), are 100% NULL values. I've read that changing these columns to SPARSE should result in a space used decrease of the table (which does have a clustered index).

Problem is, the reserved number went from 75GB to 115GB and the data number went from 40 to 80GB AFTER changing the 2 columns to SPARSE.

Can anyone explain why this is happening, i would've thought that the numbers would've gone down because now those NULL values are not stored in the database anymore?

Thanks in advance.