They overgrow, and bunny can't eat. Veterinarians have become experienced at treating this problem, so if you have a rabbit with maloccluded incisors or molars, it is best to discuss your options with your veterinarian and plan a teeth-trimming program that works for you. The condition does need to be treated and it can result in root and jaw infections, which are very difficult to reach with antibiotics. This is a perplexing problem not just limited to heavy rabbits with feet on wire. The fur on any rabbit's feet can wear down, exposing the skin and forming calluses.
Rabbits can run about on hardwood or linoleum floors without injury to their callused feet.
It's the sitting surface that matters. It must be dry. This includes all resting boards. If the litterbox is not changed daily, keep it topped with dry material so that the surface next to the feet is always dry. Moisture-damaged skin is easily cracked, allowing dirt to penetrate.