As a breeder of 'pygmy's' this often comes up - pygmy's & nigerian's were imported yonks ago and in the uk fall under that same breed society - as far as I know (show me evidence if you know better) were never kept pure with their own type. I do often have conversations with owners about one being more one way than another and vice versa - I have a very petite first kidder this year that for her size has what I would say was a very good udder and like wise another petite girl who is rearing triple boys unaided.
In my experience - sightly taller, slimmer, slightly longer in the body (longer in the body helps with kidding) v shorter, squarer, more often a longer coat, can be shorter in the body, cute but possibly kidding issues - kidding issues are not fully to do with body type, its often to do with over feeding - I learnt that the hard way.
If I ever had a min. and as kids are due to leave it would be an interesting experiment - oh and I have miked a pygmy (just for their relief) to me it's just like milking a sheep