The wheels were designed with minimum weight to reduce stress on landing gear deployment motors and on total payload by 10-20 kilos,kilos; they are 20" in diameter.
The wheel material was a bad choice: aluminium has a fast fatigue fracture potential compared to other metals. It doesn't elastically rebound into shape, but instead it memorizes stress and microscopic fractures at stress points accumulate rapidly over time. The lifetime of a good steel frame bicycle is 10 times longer than that of an aluminium one. So aluminium is not a good choice for wheels. Spirit and Opportunity had 10" Alualuminium wheels which were obviously more rigid and less burdened.
The aluminium between the ridges is 0.75mm,75 mm; the contractors who made them must have known the wheels were not suitable for long distance rocky terrain.
The small stones on marsMars are called "ventifacts" and are sharp like walking on smashed roof tiles. The design team had not expected to have such sharp ground and the rover was designed entirely for sand and pavings.
Here on earth no one would ever use 0.75mm Alu75 mm aluminium for a volcano/desert rover to do 50km 50 km (2.25mm25 mm equivalent for earth gravity). We use cable reinforced rubber, kevlar or Kevlar foam, Aluand aluminium is just about the least suitable wheel material for a cross country rover, because it memorizes stress and it'sits lack of springiness.
Tests have proven total faliurefailure at 8km8 km of all rock terrain, and limitless lifetime on sand. OpportunityThe Opportunity rover covered 42km42 km in 11 years, so they will still be able to run Curiosity for many years with limited terrain choice, and it should still be a very successful mission with a very long lifespan.
The design criteria of the wheels at the time of launch were publicised to be it'sits good traction on sand and the distance measurements using the grooves, and the JPLJPL written in morseMorse code in the grooves. The design mindset seems to have misjudged the surface of marsMars to be strangely softer and less earth-like than it actually is, and to have not done distance testing on rocky surfaces. That's born out by the fact that the wheels fail after only 8km8 km of rocky terrain. That's how long they take to be split in two all the way around.