total revenue is by definition the product of price and quantity.
TR = PxQ
If decreasing the price decreases the revenue, it means that the quantity demanded responded to the price change by less than the change in price. Prince decreases, Q increases but it does not increase as much as the decrease in price. Therefore the total revenue decreases. Decreasing the price will always have this effect on total revenue if Big winner operates on the INELASTIC portion of its demand curve