I had one runt in my litter of 11 (born 15 days ago) that was half the size of the biggest littermate that has almost caught up without any intervention. In my other litter of 9, the three chinchilla colored kits were all tiny compared to the rest (born 14 days ago) and they have almost caught up without intervention. In previous litters, there have always been tiny runts, and I haven't lost any of them, nor have I intervened. They take longer to fully grow out, but by 4 weeks, it's always been hard to tell who was half-sized at birth.
One thing I've noticed... in 3 litters by 2 does where there were chinchilla colored kits, the chinchillas were ALWAYS runty compared to the other colors. I don't know if this is something other people have noticed or had experience with, but it's been my experience. They catch up eventually, but start out wicked tiny.
If it's not baggy-skinned or lethargic, it will probably do just fine, having made it this long.