Volume is the measure of how much space an item takes up. Bulk storage containers show volume and not kg.

The volume of a single item can be determined by adding it to a bulk storage container (bulk storage bin, food storage bin) 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 of an item.

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).