Memory is not full released after deleting all QPushButtons in QTApp

[asimfile]

i am creating a program containing QPushButtons, the Problem is when i am adding QPushButtons memory(RAM) increasing but when i deleted all added buttons memory is not fully releasing.

void TempTest ::addSLOT()
{
  li= new QList<QPushButton*>;
  for(int i=0;i<50000;i++)
   {
     QPushButton *p = new QPushButton;
     p->setText("sample");
     p->setFixedSize(100,100);
     li->append(p);
   }
}
void TempTest ::removeSLOT()
{
  while(li->count())
   {
     QPushButton*p= li->at(0);
     li->removeAt(0);
     delete p;
   }
   delete li;
}

void TempTest ::addSLOT()
{
  li= new QList<QPushButton*>;
  for(int i=0;i<50000;i++)
   {
     QPushButton *p = new QPushButton;
     p->setText("sample");
     p->setFixedSize(100,100);
     li->append(p);
   }
}
void TempTest ::removeSLOT()
{
  while(li->count())
   {
     QPushButton*p= li->at(0);
     li->removeAt(0);
     delete p;
   }
   delete li;
}

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1.initialy memory is 2.0 mb

2.after adding memory increased to 49.3 mb

3.after deleting memory decreased to 4.1 mb

there is 2.1 mb of RAM it never released