Hello, when you find that the external hard drive storage space becomes smaller after being formatted, the easiest way you can have a shot is to reformat it in Disk Utility on your Mac.
You can launch Disk Utility on your Mac > click Show All Devices on the DU interface > select your USB drive and right-click it > click Erase > rename your drive and select wanted file system and scheme > click Erase to begin the format process.
When a notification jumps out and tells you the external hard drive is formatted successfully, you can check its storage space one more time.
Take care~