The charge of every shipment depends on the combination of size and weight (IATA volumetric calculation). If the volumetric weight of your shipment is bigger than the actual weight of it then the charge of a shipment is based on the volumetric weight. For example big sized packages which are actually very light.
ΙΑΤΑ volumetric weight is calculated by this formula:
Volumetric weight in kgs = (Length x Width x Height) in cm / 5000.