Mules are a breeding betwen two species of horse(equus) namely a male donkey 62(equus asinus) chromosomes and a female horse 64 chromosomes(equus caballus). A mule inherits 63 and for breeding they need to be even -so the end of the line. They can be anything from a donkey size to a draught horse size. The mixes of donkey and horse characteristics also come in a wide variety.
They often get wrongly captioned in photos as you sometimes have to take a second glance to pick out the donkeyiness(nostril flare or mane or the leg bones).
WWI paricularly saw widespread use of all sizes and shapes. The ones in Taran's illustration obviously have a bias to the donkey . Modellers seem to think they only come in donkey size but usually when as a draught animal they are sturdy beasts.