WPF Viewbox only shrink if I shrink the window horizontally

I have a grid in 开发者_C百科a Viewbox. If shrink the width of my window the grid getting smaller.. but if I shrink the height, nothing happened.

---

Sounds Like the parent container is the problem! I have solved this changing the parent container to Grid (from the default StackPanel in for instance ItemsControl.ItemsPanelTemplate).

Lets see the complete xaml!