I remember reading somewhere on this site that there are actually two different types of veileds that come from different parts of Yemen, and where they come from dictate how big their casque can get.
If im remembering correctly one comes from closer to the mountains there there is more water so their casque is shorter because they dont need to store as much water, the other comes from closer to the desert where water is harder to come by resulting is a larger casque.
Not too sure how accurate that info is but it made sense to me?