verifying a burnt usb stick against an iso image

verifying a but usb stick against an iso image

with a dvd/cd bu you can md5sum etc. as it actually is sized (as presented to os) to represent size of iso/img

but usb stick is in volumes plus free space...

so if initialise usb stick to zeroes first should be able to do a verify by appending N 0's to the signature depending on usb stick size?

am I missing something? can you already simply verify usb stick iso bu?