Wish was the one who was very behind. Do you know much about Wish's story?
My initial suspicion for the littlest one is that it may be dental. Her water consumption was absolutely massive but when she went on painkillers it is now inline with the rest of the rabbits.
They may just be stunted and not have any problem causing that other than the conditions they came from and how depleted mum was. However, we found with wish that her mouth was a complete mess- almost like she had too many teeth for her tiny mouth so they were all over the place, higgledy piggledy. With a person you would extract them and then give braces but obviously that's not an option. Wish's teeth problems showed at 16 weeks.
Now, however, she is thriving, a good weight (one we never though she would even get near) and very happy. My vet feels she does have a metabolic disorder which is another reason she is stunted (so one to watch out for maybe), but despite being a complete mini bunny, she is now just a happy bunny, just smaller than her siblings are and she will probably always stay that way.
We found oats and the dwarf excel pellets gave her the most benefit, and also very nutritious hay (right now she is eating a different kind of hay and losing weight, so hay type is important to her, for some reason).
If you don't know wish's story I can write somethig up for you if you like.