The standard formula used by most Indian couriers is (L × B × H in cm) ÷ 5000 = volumetric weight in kg. A 40×30×20 cm box works out to 4.8 kg volumetric — so even if the contents weigh 1 kg, you are billed on the higher 4.8 kg chargeable weight.
Right-sizing packaging is the cheapest freight optimisation there is. Trimming a carton by a few cm per side can drop a weight slab and lower cost on every future shipment of that SKU.