Currently, the ribbon groups are resized automatically, from right to left.

The MS Ribbon Framework specifies a property to set the order of the group reduction -- GroupSizeReductionOrder.

More info:
-- http://msdn.microsoft.com/en-us/library/ff701790.aspx
-- http://msdn.microsoft.com/en-us/library/microsoft.windows.controls.ribbon.ribbontab.groupsizereductionorder.aspx