The mig.prj is used in the MIG GUI option "Verify Pin Changes and Update Design" to validate and update the design. When using a mig.prj file created in XPS to regenerate the same design in Vivado, the C_S_AXI_DATA_WIDTH does not get generated correctly. The C_S_AXI_DATA_WIDTH changes to the default of 256 bits which can cause errors in your system if 256 is not the correct value.
To work around the issue, create a new MIG design in the Vivado tool using the same name and configuration options that you used in XPS; this will ensure the C_S_AXI_DATA_WIDTH setting is correct.
09/26/2013 - Initial release