I think it's size of horse - Jackson's poos are the largest in the field, and in fact, usually the largest we encounter when hacking. They do pile up like giant chocolate profiterole pyramids (OK that's the last time I'm eating chocolate profiteroles now that image is stuck in my head). Oh, and his diet is pretty much identical to Coop's.
I can always tell which piles the Highland pony did, too, because they're neat little ones...