Not all of Colorado was full size couplers.
The Colorado & Southern, Colorado & North Western, used 3/4 size couplers.
The Florence & Cripple Creek and Uintah Ry did as well I think.

My guess would be that the Rio Grande was either too cheap to order NG-only couplers, or was dealing with large coal trains over Marshall Pass and wanted a stronger knuckle.

Best Regards,
 Nick Gully