Volume is the measure of how much space an item takes up. Bulk storage containers (bulk storage bin, food storage bin) show volume and not kg. It is easy to get confused when looking at bulk containers volume is not equivalent to kg.
The volume of a single item can be determined by adding it to a bulk storage container and looking at the volume value.
Unique items (ex: tools, armor, weapons; non-resource items) cannot be be stored in a volume based container.
Kg is the abbreviation for kilograms. Kilograms measure the weight (also known as mass) of an item.
Liquid is also measured in grams and kilograms. A measuring jug is used to measure out precise weights of liquid. In the case of liquids, 1kg is equivalent to 1l (liter) in WU.
The weight of a single item in kg drag it to your inventory, or to a container that shows kg and not volume (typically anything but a bulk storage container).