CS6 question file sizes

Just put together a pano from Sony RX100 in CS6.  Document size as shown in the bottom lower left shows 402.9M - which is assume is about 402 MB.  When I try and save the file CS6 will not because of the 2gig limit for PSD files.  I had to save as a PSB file.  In windows explorer it is 1.9 gigs.  ???  Why is document size in CS6 showing 500 MB but the file is actually close to 2 Gig ?  All layers are visible so I am at a loss.

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Paul Amicucci