Yea it's probably similar to when you run a 32-bit vs 64-bit OS and you can only access certain amount of RAM. There comes a point where installing more RAM is pointless because the machine can't use it. 

WeAreAllSatoshi

bitcoin

Braiins OS+ on S9 miner - "Do not use cards with capacity greater than 8 GB."

Wumbo

I don't have experience with this issue with miners but it would just likely not work. I've had issues with other devices and SD cards that are to large. My guess is that the board just doesn't support larger SD cards because it is so old. I've seen this with other devices that are older.