When saving a .fig file, the entire contents of the figure must be saved for future editing, including all of the data, which in your case looks like more that 4GB. If you wish to do this, use MAT-file Version 7.3. Set this in Preferences > General > MAT-Files.
More likely you want to save just the .png image. Make sure you select "*.png" from the "Save as type" menu of the Save As dialog, or that you use the "-dpng" option of the "print" command.
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