Step-by-step explanation:
Imagine a small, fixed amount of energy like a tap. A tap could move a Pebble. But it couldn't move a Boulder.
It's the same on the molecular level. A small amount of energy could move a small molecule, but not move a large molecule. Well, this is what the Diffusion is.
A large amount of energy will move a lot of pebbles or a lot of small molecules, but a small number of rocks and a small number of large molecules.