64g U disk is displayed in the computer, about 58g, according to different brands of chips are not the same, the capacity is also displayed is not the same, but generally not more than 60, because the calculation unit of the U disk production and the default unit in the computer system is different, the production unit is 1000, the system default is 1024, So each small unit inside the difference of 24, will eventually lead to your U disk display small.
This 57g is a little bit smaller than 64g, and a lot smaller. If you are from the regular channels to purchase the more famous USB disk, then you can find the manufacturer. Ask him to replace it.
If it is not a regular brand from the community to the purchase, this is a normal situation. As long as it's okay to use, just keep it small. Because of the small brand capacity, the target is not too accurate, it is normal.