Plants need to take up water, minerals and carbon dioxide, and transport them to the leaves for photosynthesis. They then move the products of photosynthesis to where they're needed in the plant.
Larger organisms do not have sufficient absorbing area to meet their needs. Their needs are represented by their volume.
Look at these surface areas and volumes:
As their size increases, their surface area in relation to their volume decreases.
What is the surface area:volume ratio of a cube 10 cm × 10 cm × 10 cm?
0.6:1 or 0.6
The surface area = 10 cm × 10 cm × 6 = 600 cm2 (multiple by six here because a cube has six faces.)
The volume = 10 cm × 10 cm × 10 cm = 1000 cm3
The surface area: volume ratio = 600:1000 = 0.6:1
As the ratio is 0.6 to one, the one can be omitted and simply expressed as 0.6. There aren't any perfect cube-shaped organisms, but the same principles apply to small and large ones.
Once inside a larger organism, a substance will also have further to travel to diffuse.
Larger plants and animals need transport systems and to ensure that they have sufficient absorbing surface area. This could be additional surfaces, such as the digestive system and lungs in animals, or adaptations such as the flattened shape of leaves, which increase their surface area.