PowerPivot for Excel internally builds a SQL Server Analysis Services (SSAS) DB as you define and create a PowerPivot application (PowerPivot Model). The data is highly compressed (see related posts on this FAQ site regarding data compression) and resides within the .xlsx file itself in the SSAS DB format....Don't Believe Me?
Create a sample PowerPivot application in Excel 2010 and save the resulting file to your local file system. Next, rename the .xlsx file extension to .zip and save the file. Now open the resulting .zip file and navigate to the xl\customData directory. The .data file is where the PowerPivot data and thus the SSAS DB model resides...Still Don't Believe Me?
Stop an accessible SSAS 2008 R2 instance. Modify it's msmdsrv.ini file (located in the msas10_50 config directory) and place a value of 1 inside of the <IMBIMode> tags. Restart the SSAS 2008R2 instance, which should now be running in IMBI Mode.
Create and save a copy of the .data file inside of your .zip file to the local file system. Rename the .data extension to .abf (Analysis Services Backup file extension) and restore the new .abf file to the SSAS 2008R2 instance running in IMBI Mode.
You can now explore the PowerPivot application's SSAS DB and even browse (pivot) on it's data in Management Studio.
It is for these very reasons that you can use the available SSAS APIs to query PowerPivot Models because at the end of the day that is what the UI is creating.