''Hinnies r smaller because donkeys are, for the most part, smaller than horses, & equine offspring will not grow larger than the mother, in this case, a donkey, can accommodate. Hinnies do however, like mules, come in many sizes. This is because donkeys come in many sizes, from miniatures as small as 24 inches (610 mm) at the withers, to Mammoth donkey jacks & jennies that may be over 15 hands, approximately 60 inches (1524 mm) at the withers. Thus, a hinny is restricted to being about the size of the largest breed of donkey. Mules, however, have horse females as mothers, so they can be as large as the size of the largest breed of horse. There r some huge mules, mostly from work horse breeds such as the Belgian.
Other than size, there r some minor differences that occur frequently between mules & hinnies. The head of a hinny resembles that of a horse, more so than mule heads do. Hinnies often have shorter ears, although they r still longer than those of horses, & more horse-like manes & tails than mules do. They often come in horse colors, as the male parent often determines the color of the coat. Therefore, mules usually have donkey coat colors. Certain traits, like the popular gait that some horses & donkeys possess, seem to pass more readily though the male parent. Therefore, many people have tried to produce gaited hybrids by using gaited male horses on female donkeys in hopes of creating gaited hinnies.''