I noticed that my MySQL slave's DB size is smaller than the Master's DB
As tables go through many INSERT/UPDATE/DELETE cycle, mysql doesn't reclaim the space from deleted rows; instead, it internally flags the row as 'deleted' and leaves it in the tablespace. 1G size difference is not uncommon and shouldn't cause alarm.

You can try and run optimize table command on master and verify the table size get reduced. It is a good practice to routinely optimize table that does a lot of DELETE operation.