apart from cooling..the res on a piggy back shock is designed to have an expansion area, usually gas filled and seperated by a piston.
as the shaft enters the shock the oil passes from the shock into the res, this will push the piston down and compress the gas.
it allows a totally air/gas free shock so the oil wont foam up. full scale piggy back shocks do not use a baldder.

this is why rc shocks have a bladder, a small area full of air so the pressure has somewhere to go

as you say and quite rightly so...usually piggy back rc shocks dont really work very well, tbh making them work well in minature would be very costly...far out of reach of our price range...no to mention servicing them and recharging the gas and bleeding out all traces of air.
chris.