The smallest cube was the best at maximizing diffusion having been 100% diffused.
The smallest size was the most effective because there was less cell for the material to defuse in to. If the size was larger, the process would take much longer. We must take the rate and amounts our body needs these processes to be performed. On quicker scale rather than a larger one.
If cells grew to large sizes, they would not be able to diffuse through the body. Cells grow to small sizes to maximize their surface area to volume ratio.
The ratio 4:1 would be the most effective because it has a good sized surface area which is good for cells and a small volume for diffusing.
The body must have a certain amount of surface area to serve organs properly. As cells grow, the ratio decreases and also decreases the rate of gas exchange.
Because bacteria need to divide in order to keep their optimal surface area to volume ratio and therefore, can never grow that large.
As multicellular organisms, we can have specialized cells for certain functions and processes. These specialized cells perform their functions far better than an all purpose cell.