Well the donkeys have a different chromosone number than the horses so the mules have an odd number of chromosomes. If I remember correctly 62 for donkeys and 64 for horses leaving the mule with 63. This leaves the mule with out the correct number for reproduction and therefore sterile. Occasionally the genetic material does not separate like it is supposed to, and a full pair is transferred from one parent to the mule. In this case the mule will be fertile. In male mules there is no indication of how often this occurs, because the males are usually gelded at an early age to keep them from getting too stud like. Mules being stronger than horses can be much more dangerous, and seem to have a worse temperment when left intact. So the main known reproducers are the mare or molly mules, because unless spayed, these are the ones that might be able to reproduce if they have the appropriate genetic material. It may be that the reasoning behind this is that mules seem to relate more to horses than donkeys because momma was a horse. If male mules were head of a herd of wild horses, then the mares would be bred by the mule, but not impregnated and therefore reducing the growth rate of the herd by not having offspring. So actually the plan is not a bad idea to help curb the population growth, but may not work if the mule gets kicked out of head stallion position.
Donkeys come in several sizes. Mini (just about that same size as mini horses), Small standard and large standard (Pony sizes like the regular little burros that are most common) and mammoth (horse sized.) the registry registers them based on their size and breeding.
Donkey Size Categories
Miniature - up to 36"
Small Standard - 36" to 40"
Standard - 41" to 48"
Large Standard - 48" to 56"
Mammoth Jack Stock - 54" & up for Jennets, 56" and up for Jacks
A hinny is the opposite cross of a mule -- instead of a mare mother and a donkey jack daddy, a hinny is from a stallion bred to a jenny (female donkey). They are much more difficult to get since the genetic makeup of a donkey and horse are not the same (horses have more chromosomes). My understanding is the cross is much more likely to be successful when the mother has the largest number of chromosomes.
Several people have asked me for information on good books about donkeys and mules. I have listed a few here for you to look through.
Training Mules and Donkeys : A Logical appreach to Longears
The Mule Companion: A Guide to Understanding the Mule
Horses, Mules, and Ponies and How to keep them
Packin' in on Mules and Horses
The Mule Alternative : The Saddle Mule in the American West