Shipment weight calculations reflect package density. Less dense items occupy more space. Tissues, for example, take up a lot of space. Physically they may weigh 6KG, however larger carton sizes boost the chargeable weight 3 to 4 times.
Carriers charge the higher of the dimensional OR physical weight.
How To Calculate Chargeable Weight
Package in centimeters: Length*Width*Height/5000 = kilograms
Package in inches: Length*Width*Height/139 = pounds