Few times I had a problem, which actually showed smaller capacity of USB than it should be. Usually this happens on windows when I'm making USB bootable for some kind of OS.
Quick fix on windows:
Use DISKPART on Windows Vista/7/8 (Probably will work on Win 10 too)
Please note that DISKPART is very powerful and dangerous tool, if you don't know what are you doing.
Ok, so let's start. First open cmd as administrator, which you can find in control panel or typing cmd in run.

Then type this:

You will then see the following prompt: DISKPART>

to see what number your USB drive is listed as.

(if your USB is disk 3; replace # with your disk #); Diskpart will confirm that "Disk 3 is now the select disk."

(this command selects what should be the only partition on your USB drive, the small one that you want to delete to get back the larger, full partition size). Diskpart will confirm with "Partition 1 is now the selected partition."

This will delete the old partition. There are no warning prompts if you have existing data - make sure you have copied everything off before doing this!

This will create a new, full-size partition. Diskpart will confirm with the message of "Diskpart succeeded in creating the specified partition." You can type in
to confirm the new, full-size.
to leave Diskpart. You can now format your USB drive by using the standard Windows formatting process.

Notes taken from bleepingcomputer.com

If you have any problems, comment below.