The size of a package is important to consider because carriers only have so much room in their trucks to make their shipments. That means that a lighter package in a large box still takes up as much room in the truck as a large, heavy package.
The cost to ship a parcel depends on whichever is greater between the DIM weight and the actual weight of the package. If you have a five-pound item that you're shipping and that item has a dimensional weight of eight pounds, you'll be paying to ship an eight-pound parcel. However, if the parcel has a dimensional weight of eight pounds and an actual weight of 10 pounds, the shipping charge would be for a 10-pound parcel.