To buy underwear, you must first know what the fabric of this underwear is. Some fabrics are generally elastic, some are better and some are very elastic. We choose the size according to the elasticity of the fabric.
Generally speaking, you should choose underwear of a larger size. If it is better, you can choose the right one. If it is more elastic, you can choose a smaller size underwear.
But you should also choose according to your body. Fat people and thin people must buy larger underwear or smaller underwear, and those of medium height are easy to buy. If you have a size list, buy it according to the above.