When you put Grids in a StackPanel, the Grids "lose" their "star" (*) qualities since StackPanel do not fill available space. You should put your Grids in a parent Grid instead of a StackPanel (if you want to make the most of "stretching"). I typically start everything with "*" or Auto; only afterwards do I (rarely) touch up with explicit widths or heights.
It was only in wine that he laid down no limit for himself, but he did not allow himself to be confused by it. ― Confucian Analects: Rules of Confucius about his food
