DPD *******

I have used this company to collect a parcel for me which weighed about 1.425kg and fitted into a plastic bag which was little bigger than a grocery plastic shopping bag. The company has charged me over R800 for delivery from Edenvale to New Germany. I bought the exact same fabric from New Castle with additional fabric weighing 2.45kg in total which was also in a plastic bag and was charged R 250 by another courier company. When I queried the exhorbitant charge from DPD laser they said it works on volumetric size and not kgs. My question is how does 2 different companies have 2 different volumetric size for the same item? I am currently seeking advise from the Ombudsman because had the parcel been delivered to me from the company I used it would not have cost me more the the price of the goods purchased. If proof is required I will purchase the same again and see what the courier charges will be