when i use PageStack between pages then memory increase. From Main.qml, i call page MainPage.qml . after that i call page PageDetail.qml . Memory increase but i go back page MainPage.qml then memory does not decrease

this is my code

main.qml

Qt Code: 
  1. import com.nokia.symbian 1.0
  2.  
  3. Window {
  4.     id: window
  5.  
  6.     StatusBar {
  7.         id: statusBar
  8.         anchors.top: window.top
  9.     }
  10.  
  11.     PageStack {
  12.         id: pageStack
  13.         anchors { left: parent.left; right: parent.right; top: statusBar.bottom; bottom: toolBar.top }
  14.     }
  15.  
  16.     ToolBar {
  17.         id: toolBar
  18.         anchors.bottom: window.bottom
  19.         tools: ToolBarLayout {
  20.             id: toolBarLayout
  21.             ToolButton {
  22.                 flat: true
  23.                 iconSource: "toolbar-back"
  24.                 onClicked: pageStack.depth <= 1 ? Qt.quit() : pageStack.pop()
  25.             }
  26.         }
  27.     }
  28.  
  29.     Component.onCompleted: {
  30.         pageStack.push(Qt.resolvedUrl("MainPage.qml"))
  31.  
  32.     }
  33. }
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MainPage.qml

Qt Code: 
  1. import QtQuick 1.0
  2. import com.nokia.symbian 1.0
  3.  
  4. Page {
  5.     id: mainPage
  6.     Text {
  7.         id: text1
  8.         anchors.centerIn: parent
  9.         text: qsTr("Hello world!")
  10.         color: platformStyle.colorNormalLight
  11.         font.pixelSize: 20
  12.     }
  13.  
  14.     Button{
  15.         width: 150
  16.         text :"page 2"
  17.         anchors.horizontalCenter: parent.horizontalCenter
  18.         anchors.top: text1.bottom
  19.         anchors.topMargin: 10
  20.         onClicked: {
  21.  
  22.             pageStack.push(Qt.resolvedUrl("PageDetail.qml"))
  23.         }
  24.     }
  25. }
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PageDetail.qml

i
Qt Code: 
  1. mport QtQuick 1.0
  2. import com.nokia.symbian 1.0
  3.  
  4. Page {
  5.     Button {
  6.         id: button1
  7.         x: 142
  8.         y: 437
  9.         text: "Tro ve"
  10.         onClicked: pageStack.pop()
  11.     }
  12. }
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how to decrease memory when go back page ManiPage.qml ?
thanks.