Actually they’re quite large files. If you actually save the picture to your computer you can then zoom right in to the QRs. It’s Tumblr that doesn’t let the pictures be shown full sized. Alternatively, if you have a smart phone or tablet you can zoom in on the pictures without saving them.
Now I understand you’re frustrated but it’s extremely rude to go round accusing people of things in a derogatory way. So to answer the accusation, no I don’t make the files tiny because I’m scared someone will “steal” them. They’re tiny because tumblr can’t show them their accurate size.
Hope you can get them to work! I appreciate you wanting to use them.