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On cars, smaller temporary spares are commonly used. Handling and steering are sub-standard with them on. But they usually put full-diameter spares on pickups, as they might be loaded or towing when they get their flat.

The diameter of the wheel has little to do with it. It is the overall diameter of the tire that matters. My guess is that the spare has taller sidewalls (1/2 inch per side) so the overall rolling diameter is the same.

They'd do this for cost reasons. You might get the pretty 17s on the truck with a cheaper steel-wheeled 16-inch spare. Spares haven't been part of the regular tire rotation schedule for years, so this isn't a problem.

If my overall diameter theory checks out, I wouldn't worry about it. You could check with a tape measure or post the complete tire size of a standard tire and the spare here so we can compare them.

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The rim size makes no difference as GM cheaps out on the spare rims using cheap steel rims of the smallest size 6 bolt pattern on their spares. You will also notice that the tire brand don't match also. However, the overall tire size will. If you're wondering why GM cheaped out on this you can figure it this way. When shopping for the truck no one bothers to look at the spare. Second if they can save $20 using this combo and selling a million trucks a year...well you can do the math.

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