WPF Grid Star sizing does not work with size sharing groups

To simply quote and link: Unfortunately, this is where the SharedSizeGroup method breaks down. If you want to have a shared Grid that uses the whole available width and automatically adjusts when that space changes you're going to need a different method. A column set to * in a shared group acts just like an Auto column and won't fill or stay within the given space. Taken from John Bowen's blog.