How many species of fish are their?

Smaller molecules diffuse easily as compared to the larger molecules. In other words, smaller is the size of molecule, easier and faster is its diffusion across a semipermeable membrane.

Diffusion is a process by which molecules move ( via semipermeable membrane) from an area where its concentration is higher to an area where its concentration is lower.

Size of molecules can impact the process of diffusion as smaller is the size, faster is the rate of diffusion.