Life is more relaxing when you tower over the rest of the community. Large animals can run to escape predators, if predators dare to attack. They can reach the higher foliage of tall trees, or can knock them over, for that matter. Food stays in the intestinal tract longer than in small animals, so giants can digest older and tougher plants more efficiently. Pound for pound, large creatures do not need to eat as much food;smaller animals sometimes must eat the equivalent of their body weight every day. Because they can store a great deal of food energy, large animals can remain nocturnal for longer periods of time, and can even go for days without eating. Their brains are larger so they have more intelligence potential, but thinking is very energy-draining. Ability to withstand long journeys allows large animals to migrate when local conditions become unfavourable. Finally, large animals live longer lives...some tortoises are hundreds of years old!
Giant plants benefit because their seeds can be caught in the higher altitude winds, so they can be carried further. Moreover, the higher they grow, the more of a monopoly they have over the sunlight. In general, larger plants tend to survive stressful conditions better than small ones.