I wouldn't say that it is extremely bad with the weight difference, but I would watch them a little more closely, especially the smaller one. Try to pay attention to how much his is eating and drinking. Also, if you can, if he is passing his food alright. If it is pretty low, and he is not gaining weight or starts to lose, bring him to a small animal doctor.
Just to say, I'm not a vet but I have had some rabbits and other rodents. So just keep an eye on him.