Can you put the new bun in a separate room? It’s important for him to become familiar with his new surroundings before potentially stressing him out with the smell and sight of a new bun. I know it sounds silly, but it can stress them out. It’s good you haven’t introduced them yet. Are they both spayed/neutered? Bunnies can’t properly bond without being fixed. After he has settled in for at least a month, then you can start swapping scents and prebonding, as well as have him in the same room. This will really help to make sure he has become comfortable. After you’ve prebonded for another month, then you can start sessions on neutral territory. If no where is neutral in your house, clean a specific area with vinegar to remove the smell of either bunny, and keep it bunny free until you start sessions. Start sessions slowly with 10 minutes.
The size difference doesn’t matter. It all comes down to their personalities and how vigilant you can be during sessions to prevent any scuffles. hope this helped 