Factory fault

I bought a pack of boyleg large underwear at one of your branches it was a very good fit and i was very happy about it so i decided to buy another pack but different patterns still boyleg large and it was a complete horror and way too small it feels like its a small not even a medium. It has no stretch and its very uncomfortable. I tried to complain on your app but was told i cant return underwear but obviously its a factory fault because why is the one pack sitting different from the other pack. How must i trust your product by just saying yes it is my size when in fact i now dont know what my size is according to what sizes mr price has. So please contact me as soon as possible with a better response than just i cant return underwear because that is unacceptable as ive just wasted money on the second pack of underwear