In short:
Granite = Short 3S wheelbase / chassis, standard arms, wide wheels
Big Rock = Long 3S wheelbase / chassis, wide arms, narrow wheels
Pictures paint a thousand words etc...
left to right..
Green = Long wheel base 3S chassis / wide arms / Big rock wheels / Kraton Body
Middle blue = Short wheel base 3S chassis / standard arms / Granite wheels / Granite body
Right Blue = Long wheel base 3S chassis / wide arms / Granite wheels / Big Rock body