iOS NSMutableArray removeObjectAtIndex

This is my code in ViewController.m file,

- (void)viewDidAppear:(BOOL)animated
 {
   [super viewDidAppear:animated];

   NSLog(@"%f",[self usedMemory]);
   NSMutableArray *array= [[NSMutableArray alloc]init];
   for (int i = 0; i < 100; i++) {
      NSMutableData *data = [NSMutableData dataWithLength:10000];
       [array addObject:data];
   }
   NSLog(@"%f",[self usedMemory]);

   for (int i = 0; i < 100; i++) {
       [array removeObjectAtIndex:0];
   }
   NSLog(@"%f",[self usedMemory]);
}

Here is the conslut result:

2015-01-26 22:39:00.058 audio_memory_test[9050:874963] 25.011719
2015-01-26 22:39:00.060 audio_memory_test[9050:874963] 26.312500
2015-01-26 22:39:00.060 audio_memory_test[9050:874963] 26.312500

My question is ： Why delete objects in the array, the memory has not been released ? and what removeObjectAtIndexmethod has done? How to do can release memory?