Constriction of pupil ( miosis ) mediated via sphincter muscle of iris (sphincter pupillae) . It is innervated by parasympathetic nerves . Dilation of pupil caused by radial muscle of iris and it is innervated by sympathetic nerves 

presynaptic Parasympathetic fibers for sphincter puillae originate in the Edinger- Westphal nucleus in the brain stem and travel on the surface of occulomoter nerve to cilliary ganglion . Short cilliary nerves (postsynaptic nerve ) leave the cilliary ganglion and innervate the sphincter pupillae and cilliary muscles 

Extradural haematoma is a collection of blood in between dura and the skull. Extra dural heamatoma exert pressure on brain mater and compress the  brain  matter because  of skull is rigid bone. Initially this pressure exert on side of lesion and later it extend into other side. This pressure act on occulomoter nerve and compress the superficial parasympatheritc fibre of the affected side.Therefore parasympathetic nerve innervation for sphincter pupiller lost and it leads to unilateral dilated pupil.