How it works:
When a creature (or part thereof) is butchered, the game divides the creature into each of its component parts, then looks at the size of each part (determined by taking the creature's total body size and then dividing it up with each body part's relsize determining its share). Each part with a weight of less than (approximately) 185 is discarded. Parts which have a size of more than (approximately) 185 give butchering returns.
For each part that gives butchering returns, DF examines the tissue layers of that part and looks at the materials those tissues are made from. Materials which have the BUTCHER_SPECIAL tag yield stacks of items, with each body part giving one or more item that is added to the stack. Bones, teeth and shells also generate stacks, despite not having the BUTCHER_SPECIAL tag. Materials which don't have BUTCHER_SPECIAL, such as skin, chitin, scale, nervous tissue, cartilage, hair, etc, yield one item per butchering job, no matter how many body parts are being processed. Body parts with the TOTEMABLE tag, such as skulls, are handled differently, and always drop a totem item no matter how small or large the creature was.
When you butcher a corpse, you'll get at most one of each non-meat/fat/bone material in the corpse per butchering job. If you manage to slice a creature into multiple parts, each one of those can give you a skin or chitin or whatever, provided the parts you're butchering are large enough to give anything at all. So cutting a buzzard into many pieces isn't going to give you much more than the intact animal would, but splattering an elephant into individual limbs can be very worthwhile for generating elephant leather.
It is possible to mod the material files to cause skin and chitin to drop in stacks like any other meat, but it's not very useful. The tannery reactions don't respect stack size - an entire stack of raw skin will be processed into a single piece of leather.