Thanks for trying MonoBrick. I don’t know why this is the case. My best guess would be to try to download one of the other sizes.
I used the 512MB firmware image. Is there any difference between that and the larger images?
No they are identical but will if you use a small image for a large SD card you will not be able to use all the remaining space out of the box. So you should go with the one that is close to the size of your SD card.
Let me know how it goes