I could solve the issue with the help of the Telerik guys:
This issue is caused by the ListBox behavior and specifically its ItemsPanel. In order to update the size of the ListBox along with the size of its content you can set the ListBox.ItemsPanel to a StackPanel.
<ListBox.ItemsPanel>
<ItemsPanelTemplate>
<StackPanel />
</ItemsPanelTemplate>
</ListBox.ItemsPanel>
Now the Listbox resizes along with its content.