SGA size not decreasing after reducing memory_target (Oracle 11.2.0.1)

I have an Oracle 11.2.0.1 database using dynamic memory allocation with parameter MEMORY_TARGET. When I decrease it with SCOPE=BOTH, the size of the SGA doesn't seem to decrease along with it. I have tested this on multiple systems. Here is an example

SQL> show parameter memory_targetNAME                                 TYPE        VALUE------------------------------------ ----------- -------------------------memory_target                        big integer 820MSQL> sho sgaTotal System Global Area  855982080 bytesFixed Size                  2180544 bytesVariable Size             520096320 bytesDatabase Buffers          327155712 bytesRedo Buffers                6549504 bytesSQL> alter system set memory_Target=720M scope=both;System altered.SQL> show sgaTotal System Global Area  855982080 bytesFixed Size                  2180544 bytesVariable Size             520096320 bytesDatabase Buffers          327155712 bytesRedo Buffers                6549504 bytesSQL> show parameter memory_target;NAME                                 TYPE        VALUE------------------------------------ ----------- -------------------------memory_target                        big integer 720MSQL>

However, after I bounce the instance, the SGA seems to size itself appropriately.

SQL> sho sgaTotal System Global Area  751595520 bytesFixed Size                  2179536 bytesVariable Size             415239728 bytesDatabase Buffers          327155712 bytesRedo Buffers                7020544 bytesSQL>