Diffusion is a process, not a substance. It's the movement of any kind of molecule from an area of high concentration to an area of low concentration. So, diffusion can involve both small and large molecules.

Here's a breakdown:

* Small molecules: These diffuse faster because they have less mass and are more easily jostled by collisions with other molecules. Examples include oxygen, carbon dioxide, and water.

* Large molecules: These diffuse slower because they have more mass and require more energy to move. Examples include proteins, carbohydrates, and lipids.

The size of the molecule is just one factor affecting diffusion rate. Others include:

* Temperature: Higher temperature means faster diffusion.

* Concentration gradient: The larger the difference in concentration between two areas, the faster the diffusion.

* Medium: Diffusion is faster in gases than in liquids and slowest in solids.

Let me know if you have any other questions!