The opening scene shows the stagecoach pulled by six horses but when the stage arrives in the town it has only four horses pulling it.
Not only has he gained inches around the middle, but he has lost a lot of inches in height.
When Cheyenne is about to be tortured by hanging him over a fire, you can see it's a stand-in on the long shots. You can also see this when he is lifted up over the fire. (Either that or Cheyenne has gained a few inches around the middle since breakfast that morning.)
Before Cheyenne (Clint Walker) & Wes McQueen (Don Megowan) start out, they're on their horses talking. They are obviously on a sound stage, with a fake western backdrop behind them, because Don Megowan casts a very visible shadow onto the backdrop, showing that they aren't really in the desert.
When Cheyenne (Clint Walker) & Wes McQueen (Don Megowan) are on their way to find Billy McQueen (Tim Considine), there's a long shot of them on their horses. For a second, there's a third mystery horse viable, out in front.