People using the forum to sell their products are not posting to help they are posting to make money. Thus they are not telling the truth.
Flash drives that change size can have a variety of causes. Counterfeit drive, reformated wrong by the user (FAT32 only supports drives up to 32 GB), partitioned by apps the user is testing, etc. And if I’m not mistaken the Windows Install app partitions the USB drive to protect the OS software since Windows can only access the 1st partition of removable drives.
So the issue with these flash drives is clearly users who don’t know what they are doing or what the app they are using is doing.
The youtube link you posted is good for fixing partitioned drives just be very careful selecting the Drive to Clean. Diskpart works on the machine’s hard drive also. 
BTW What did you find was using all your space on your drive?
