Why does my Xbox Captures take as much as a lot storage on a USB drive in comparison with OneDrive

I’m not an knowledgeable on these things however for some cause my Xbox captures take up a lot storage when uploaded to a USB drive in comparison with once I uploaded them to OneDrive. I need to save all my captures earlier than they get deleted, however I don’t need to pay cash for additional OneDrive storage, so I’ve been attempting to add them to USB drives however they take up an excessive amount of area on the drive. Thanks for all assist!