Usual practice is to fit the smaller tyres on like that, it's to do with soil types and moisture. The wheels are filled to eleven o'clock position with a water antifreeze mix for additional weight, then the outside wheels act as additional ballast for traction, if the wheels start to sink into the ground in a soft patch then the extra tyre, contact adds flotation to support the tractor.
Also the steering is better as the smaller wheels on the outside are not causing the tyres to squirm in a tight turn.
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