Why are firearms with projectiles under 20 millimetres usually classified as small arms?

In military science, the classification of weapons is primarily distinguished by their size and intended use. The term small arms refers to portable firearms designed to be operated by an individual, such as rifles, pistols, and machine guns.123

In contrast, artillery is defined as crew-served weaponry, including big guns, howitzers, and mortars, which possess a calibre greater than that of small arms. While the 20-millimetre threshold is a common convention used to differentiate these categories, the fundamental distinction lies in the weapon's portability and its role on the battlefield: small arms are intended for individual infantry use, whereas artillery pieces are larger, crew-operated systems designed to fire heavier projectiles, often over longer distances or in specialized trajectories.41