Perfectly cooked jumbo shrimps and jumbo prawns have only JUST turned opaque (the flesh of the shrimps has turned white and opaque) and they have JUST curled up in shape and become firm. Any cooking of your jumbo shrimps or prawns beyond this point is overcooking them and they will taste worse and worse. As you continue to cook or grill jumbo shrimps or jumbo prawns beyond this magic point of perfection they will start to curl up even tighter and tighter as they get tough in texture and lose flavor. The over-cooked jumbo shrimps or prawns will also tend to shrink and shrivel. If the prawns had a lot of water weight when raw, when over-cooked, the shrimps may shrink down to half their size or even a whole lot smaller. If your grilled jumbo prawns end up tough, dry, and 1/4 the size they were when raw, then it is a 100% certainty that you have over-cooked them.