When a package is relatively large for its weight, i.e., when the result of multiplying its three dimensions and dividing by 139 is greater than the weight, then this result will be used as the weight for the purpose of price calculations. Please visit the following link
Dim Weight
http://www.dhl-usa.com/en/tools/volumetric_weight_express.html
https://pages.message.fedex.com/weight_calculator/
http://www.ups.com/content/us/en/resources/ship/packaging/dim_weight.html