Recently I have had some USB sticks which volumes shrank.
These are some of the snapshots of how my USB drive looks like.
It is actually supposed to be 16 GB drive, however showing wrong size.
If you have similar issue, read on.
Below is how it shows on My PC.

Below is how it shows on disk management.

Below is a fix to USB drives showing wrong size.
1) Open Command Prompt as an administrator. Once DISKPART is opened, type in below commands colored in RED
.
2) diskpart
3)
list disk
you will see a list of disks currently attached. Its very important you have to make sure you select the right disk.
4) in my case I had to type : select Disk 2
5) clean
6) create partition primary
7) list partition
8) format fs=NTFS
(let it complete formatting, it normally takes a few minutes)
Once done, it will show correct disk size of your USB.
