Almost everyone experienced with peanuts and the double dwarf gene will say there is no way a peanut can survive past the first week or two because of genetic problems inherent with the deformity. But then there are people that swear they have peanuts that have survived several weeks. So who knows, maybe you do have a peanut there and it's just one of those rare few that survive a little longer than the others.
I haven't ever had the experience with a peanut, but I have had a dwarf baby that was a runt, and at 8 weeks she was more than half the size of her siblings, thin, with a pot belly, but behaved normally. When I got her I changed her diet and after that she started gaining weight, lost the pot belly, and eventually caught up to the right size for her breed. Ended up being even a little bigger than normal for her breed, and at 8 is still very healthy. With her I figured it was either a nutritional deficit from poor quality feed or an intestinal parasite issue causing the smaller size, wasting of the back and hindquarters, and pot belly appearance; and that one of these was the cause of her being underweight and so much smaller than her siblings. All I could ever figure is that the high grass hay diet that I started her on, then gradually introducing a high quality feed into her diet, helped correct whatever was wrong and causing the failure to thrive.
I've also had a friend that had a similar problem with a runt that was much smaller than his siblings, and just not growing and thriving like them. She was feeding a good diet already, so she tried addressing a possible intestinal parasite issue as the cause of the failure to thrive, and in the next several weeks following treatment, the baby started gaining weight better and eventually caught up in size to his siblings. I know of one other person that had a runt with failure to thrive who treated for possible parasites as the cause using fenbendazole, and had the same results. The baby got better and caught up in size.
So I'm just sharing these other stories in case you actually have a runt with a so called 'failure to thrive', and if you are wondering if there may be a treatable cause for your rabbits smaller size, that there actually might be possible solutions to the problem. I can't say what exactly for your baby, but just that there could be a treatment.