Hi Sreenu
When your are checking the table size in transaction code DB02 or ST04(Only in MS Sql DB you can use the transcation) "table size = data used + Unused space", at the same time when your are executing the Query in Sql Query it will show only data used space. I think in your case also same think physical size of the table space 17 Mb from the query its showing 15 Mb 2 mb may be unused space.
Refer the MS Sql DB using the transaction ST04 or DB02 you can check the table detail.
Regards
SS