Ah, so it's their negligible volume (and the fact that the difference between the volume of smaller atoms and larger ones is negligible) that makes Avogadro's law an apt approximation.
Thanks.
Avogadro's Law states that the same volume of two different gases (at the same temperature and pressure) contains the same number of particles. I understand that this hypothesis was first suggested after Gay-Lussac's experiments with electrolysis showed that the volumes of hydrogen and oxygen...