What is the difference between life processes in large and small multicellular organisms?

What is the difference between life processes in large and small multicellular organisms? Why is the complication of the circulatory system closely related to the progress of the respiratory system?

1) As a rule, large organisms are more specialized and complex, because it is more difficult to maintain consistency in a large system than in a small one. Also, in small animals, the processes are more intensive, the metabolism is accelerated.

2) Oxygen enters the bloodstream from the respiratory system. The better one system is developed, the better the other is also developed. In the most primitive animals, oxygen diffusely penetrates through the integument, and they do not have a circulatory system (for example, in flatworms). The evolution of the circulatory and respiratory systems always goes together.