memory engine not release memory in linux

[zhang maosen]

1. mysql version 5.5
2. linux 2.6.18-92.el5

I have create 300 tables engine is heap, the size of each table is about 5MB.
And I use 'ps aux|grep mysqld' and mysqld have used about 1.8GB mem,
When I drop these 300 tables, and use 'ps axu|grep mysql' again, mysqld also have used about 1.8GB mem.
why? Thanks

best
wishes

zhangmaosen