VertiPaq Analyzer is useful to analyze VertiPaq storage structures for a data model in Analysis Services Tabular.
A complete description and short tutorial is available in the article Data Model Size with VertiPaq Analyzer.
You can analyze a Power Pivot data model by restoring it in Analysis Services.
The current version is not compatible with the Power BI Designer data model.
UPDATE 2015-09-27: Version 1.3 now handles correctly renamed tables and renamed columns in the data model.