Thanks for your response. Can you tell me what options to check to reduce the size? See attached picture.
All you need to do is build the footprints. Simply use the ID tool to see the pixel values of the white space in your mosaic dataset and then adjust the settings in the build footprints tool. By default they are set to eliminate the 0 and 255 pixels. If your rasters have additional values then adjust the min or max values.
You should not have to resample the rasters for this workflow.
Its a pity the Raster to polygon doesn't work as it should.
Actually please ignore me I just read you comment further up.
How about using the 'Build Boundary' tool on your mosaic dataset?
http://resources.arcgis.com/en/help/main/10.1/index.html#/Build_Boundary/001700000086000000/