Taking what the Reddit user said, I made my own investiagations.
The file sizes for Birthright and Conquest are 1634 MB and 1624 MB respectively.
Now, the 3DS follows a memory block system,
128kb = 1 block
1mb = 8 blocks
1gb = 8,192 blocks
So 1634 MB is 13,702 blocks and 1624 MB is 12,992 blocks.
Together they make up 26,694 blocks. Why does this matter? The file size of the Japanese version of Fates is at
24,666 blocks That’s less than the US version.
Either Amie is here to stay or maybe the larger file size is due to the possibility that dual audio is included in the US version of Fates.