All international carriers base their shipping rates on the greater of Actual Weight and Dimensional Weight.

Actual weight is the weight of the package when put on a scale. Dimensional weight is based on the size of the package. 

Large items that have a low Actual Weight relative to their size-- like pillows and lamp shades, will result in a larger Dimensional Weight. Some people refer to this as “volumetric weight.”

Dimensional Weight is calculated by multiplying the dimensions of the item. Length multiplied by Width multiplied by Height, then dividing that total by the "DIM Factor." The industry standard DIM Factor is 139. 

So, the formula is:

Dimensional Weight (chargeable pounds)  = Length x Width x Height (inches) / 139

Oversized shipments are defined as linear dimensions (total Length + Width + Height) greater than 80 inches (203 cm).