Even on a recently molted jumbo prawn or jumbo shrimp, because the morphology of the jumbo prawn the jumbo prawn telson and uropods may be thicker than the rest of the jumbo prawn shell - particularly where they join the main true body segments of the prawn.
Those of us who want it in lay terms: the grilled jumbo prawn tail can be tougher than the main shell and legs!
Therefore, if your recently molted and newly grilled jumbo prawns have thick tails but thin shells, just don't eat the tails! Use your own judgement on this one, but remember that the more shells your eat the more calcium you'll get.