This is sad

Breed ever-larger & ever-stronger donkeys in response to the increasing load.

This is such a tragic comedy.

That looks more like a skin disease than a friction abrasion.

When we rode the mules at the Grand Canyon, they had a weight limit, and when you checked in, they had a scale to enforce it.

This is from poorly fitting tack more than anything else. A saddle has an internal structure (called a tree) that keeps the weight off the animal's spine and distributes it across the sides of the back. A bad saddle will rest on the spine. This will be immediately apparent, since the increased pressure will prevent the sweat glands from working--there will be dry spots where there is too much pressure. It is very easy to see. This poor donkey's back is caused by the repeated use of an ill-fitting saddle. Yes, overweight people will contribute more to the damage to the animal's back, but even a light person would hurt this donkey with the tack it is wearing. Fat people cause back/joint issues in animals that is internal, not readily visible like this wound is. It's sad, but a misleading picture. Yes, animals shouldn't be overloaded (carry no more than 25% of their body weight, including tack). But animals also need properly fitting tack. It's like having shoes that don't fit.