I just want to clarify, you're looking to setup table partitioning within the schema of the database to multiple filegroups, correct? The reason I'm trying to clarify is because I've never seen an estimate for storage before, are you using a wizard?
Additionally, is there an index on the field you're looking to partition across? I'm wondering if the statistics are just wildly off. If there's an index on that field, UPDATE STATISTICS WITH FULLSCAN the table and see what kind of estimates you get.
Finally, and I hope you're on a dev system, just chuck the estimates out and do your partition. Then go to the dm_ views and check your partition sizes with actuals. Estimates are notorious for being wrong.
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