Because a measure of mass is desired in the set of standard units.
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The SI unit for mass is kilograms Grams or Millilters
Most of the world uses the SI system of measurements so weight is measured in grams, kilograms or tonnes. In the US the old ounces, pound and tons (Imoerial) system is used
Density In SI, density is measured in kilograms per cubic metre.
Density is measured in units of mass divided by units of volume. The SI unit is kilograms/meter3.
Not sure what you mean with "light build". The SI (metric) unit for mass is the kilogram. For small objects, smaller units such as grams and milligrams can be used; for larger objects, the ton (= 1000 kilograms) is also used.