From the blog:
Wouldn't it be great if you would be able to control the size of each particular item in your grid? To break out from dullness of having all items the same size, like, for example, to emphasize featured items; or for just any other reason?
And it turned out there was. Or at least it would be, if Windows 8 XAML supported Style Bindings like Silverlight 5. Not having that, the solution required that extra step that would allow binding to properties from within declared styles.
http://www.tozon.info/blog/post/2012/09/01/Variable-sized-grid-items-in-Windows-8-apps.aspx