The Slim Head is a more realistic head than the original head. Apart from being slimmer of course the slim head has a more pronouced chin. The default circumference of the slim head is 20 inches (51cm), but you can specify the circumference to be anything you want from 15 inches (38cm) up to 33 inches (84cm).
Since the neck of the slim head is narrower than the regular neck it includes a neck stiffener for cuddle and travel models.