I have found that when creating a new world in Minecraft Realms, the LargeBiomes generation option doesn't work. I selected LargeBiomes before creating the world, but the world I came into was default generation. The reason I know this is because I played on the same seed in a single player world with LargeBiomes, and the spawn looked completely different. I also looked up the seed on Chunkbase.com and found that the world is indeed a default one.
Steps to reproduce:
1. Create a new world in single player, with a seed and default world type
2. Create a new world in single-player, with the same seed and Large Biomes
3. Create a new world in Realms, with the same seed and Large Biomes
4. Watch how the Realms world is identical to the default world and not the LargeBiomes world
Can second that this appears to be a consistent issue, created two realms set to LargeBiomes, world generated as default.