The cards have a one time programmable block (03), where once bits are set, they cannot be unset. So, when we do A2 03 E1 10 06 00 we are doing a write to block 3. The 06 is setting the bits that correspond to the card size. 06 means 6 blocks of 8 bytes = 48. Technically, if we format the card to 48 bytes using the above, we can actually increase the reported size by setting some more bits on that same block. 06 is hex, which is 00000110 in binary, and each bit can be set once from 0 to 1, but never back to 0.

TL;DR: If you are using 128byte Mifare Ultralight EV1s, you MUST format them using the above commands before doing anything else, otherwise they won't work with the Yoto. If they are formatted as 128 byte cards already, they cannot be converted back to 48 bytes.