The size of underwear has nothing to do with the thickness of underwear, but if it is 34A and the coaster is very thick, you can buy 34B. There is a girl in our dormitory.

So if your underwear is not particularly thick, you don't need to buy a small size, but I personally suggest that you don't buy underwear online at night unless you have chosen a strong one. Girls' breasts are very delicate, and different brands of underwear have different sizes. Maybe this one suits you, and the other one suits 34B. Maybe you should wear that 34B adjustable underwear, but the other one is just 34B with a thin cup. I hope I can help you.