ExportToImage returnImageSize & dpi parameters

Say I have a Diagram that is 1920x1080.  I call ExportToImage with an enclosingBounds of (0, 0, 1920, 1080), a returnImageSize of (1920,1080), and a dpi of 384.

 I get an image that is 7680 x 4320.  Is that expected?  I am (perhaps naively) expecting to get an image that is 1920x1080, based on the value of the returnImageSize parameter.

 Thanks!

-David