Right now, I have one in the grow up cage that is really tiny. I've had some small ones that died when they were in the fuzzy/hopper range, so I sort of expected it with this one too. She seems pretty healthy, though. The only issue at all is that her fur seems a little thin, like she only grew 2 out of every 3 hairs she should have or something. It is still soft, smooth, etc, but I can see her skin through it a little.
I like her, she is very cute. And she has grown some. When she was the age to be weaned, she was so lightweight I could hardly feel the weight of her in my hand. My daughter said it was like holding a feather. That was at least 2 weeks ago. Now she still weighs less than an average weanling, but at least it feels like there is something on my hand when I hold her.
So basically I'm just watching her to see what happens. Maybe they are dwarfs. Maybe they just got a slow start on life for some reason and will eventually catch up.