It’s not the size of the cart, the U2+ can play both Briley Witch games and they are both over 512k in size. It’s almost certainly going to be the device doesn’t yet support the MD2 bank switching configuration.
Viewing post in Kingdom of the Seven Stones comments
Interesting. I looked at the firmware code, and I see that the the max rom capacity for the U2+ is 1MB, and they do support Magic Desk but the comment says that it supports a max of 16 x 16K chunks, which means 256K total. So they do support some banking but not the full range. Probably a very easy fix.