With Q1 2013 we have introduced dynamic FontStyle, FontSize and Brushes for Windows8 and Windows8Touch Themes.
For example with the Windows8Theme and Windows8TouchTheme applied, the size of the GridView's containers (for example GridViewCell, GridViewHeaderCell, GridViewFooterCell etc.) will be adjusted based on the size of the content to be shown in them. If you set a CellTemplate for the column, having larger content occupying the entire cell, then the container showing it will be larger as well.
You can compare:
The appearance before:
The appearance now:
In order to have a different (smaller) Size for the containers, you can define a Style targeting the element you are interested in. Then set its Height property with the desired value. In the example with the CellTemplate, you could define a simple Style targeting the GridViewRow element and set a fixed Height.