The designer always defaults back to pointSize even if the Text element is set to pixelSize. And it will even add pointSize to the element if pixelSize is already present.

Create default Hello world app.
Set the font in design view to pixels and some size. This will add the line to the Text element, can be seen by switching to code view. When switching back to the designer view, the font size is on points again. Now increase the size only. This results in having both the pixelSize AND the pointSize in the Text element.

Text {
   text: qsTr("Hello World")
   font.pointSize: 13
   font.pixelSize: 16
   anchors.centerIn: parent
}

Please let the text size field of designer follow what is in the QML code.
Issue seems to exist longer (also in 2.7)