how does MERGE command work

Hello, I am trying to explain a sizing discrepancy in two of my tables. For one of them the average row length is 101 and the number of rows is 362mil. Its size - calculated as select sum(bytes) from dba_segments is about 150G. The other table has average row length 33, the number of rows 16mil and the size of about 40G. When I use oracle create_table_cost procedure, the size estimates that I see are  40G (instead of 150G) for the table1 and 900M (instead of 40G) for the table2. I also did a quick calculation just multiplying the record lenght by the number