Why did the Soviet Union think a lightweight, automatic rifle was needed?

Now he chats with his plain clothes rebel comrades next to a gun-mounted pick-up truck, hoisting a Belgian-made FN light automatic rifle that's half his height.

The XM29 was intended to be a shoulder-fired, semi-automatic grenade launcher in 20 mm calibre and to incorporate a compact, lightweight 5.56 mm rifle as a back-up weapon.