From the pics, Mary appears to have much fuller, thicker hair, and her roots and scalp are probably used to the weight of such thick, long hair. Julie, on the other hand, appears to have fine, wispy hair, and probably a more sensitive scalp. Having seen an IRL hairpulling contest beween two long-haired, evenly matched, 30-somethings, I'd say Mary will be able to pull more hair from Julie's head.