Why does my usb have less space than it says?

Here’s a straightforward, no-fuss way to use Disk Management in Windows to fix the “Why does USB have less space than it says” issue.
1. Open Disk Management  
Press Windows key + X, then select Disk Management.
Alternatively, press Windows key + R, type diskmgmt.msc, and hit Enter.

2. Find your USB drive  
Look for your USB drive in the list at the bottom.
It might be labeled as Removable and will show the current partition and size.

3. Check for unallocated space  
If you see a big chunk of "Unallocated" space on the drive (that’s the missing capacity), that’s the issue.

4. Delete existing partition(s) 
Right-click on the partition(s) of your USB drive and select Delete Volume.
Confirm when prompted.
Warning: This deletes all data on those partitions!

5. Create a new partition that uses the full drive  
After deleting, the whole drive should now show as Unallocated space equal to the full capacity.
Right-click on the Unallocated space, select New Simple Volume.
Follow the wizard:
Click Next
Set the Volume size to the maximum (default)
Assign a drive letter
Choose File System (NTFS or exFAT)
Check Perform a quick format
Click Finish

6. Wait for the formatting to complete  
Once done, your drive should now show the full capacity in Windows Explorer.

Hope the above sharing can help you solve the problem of "Why does USB have less space than it says".