Denny Lee from the SQL Customer Advisory Team and Dave Wickert from the Analysis Services Team have started a PowerPivot blog named the PowerPivot Twins and created a post describing the inner workings of a Excel PowerPivot workbook. Using the steps described in the blog post you can find the tables and columns that are taking up alot of space within your Excel PowerPivot workbook and can potentially remove any columns that are not required.
My guess is were gonna have to use this pretty often, especially regarding the 2GB file limit of SharePoint.