storage during migration

CANI GET AN ESTIMATE OF HOW DATABASE SIZE(STORAGE GB REQUIRED) CHANGES WHEN THE SIEBEL DATA IS MIGRATED FROM DB2 V 7.1 in MAINFRAME TO ORACLE9i in DISTRIBUTED SAN?

IS THERE A STORAGE DIFFERENCE FOR THE EXACT SAME DATA DUE TO ARCHITECTURAL DIFFERENCE ?

I WOULD LIKE TO KNAOW WHETHER THERE ARE ANY ADDITIONAL OVERHEADS ON THE ORACLE SIDE WHILE PLANNING TO MIGRATE?