Newbie: advice re: extra small chick

I bought orpington chicken eggs for my first go at incubating and hatching. Had a very low hatch rate and now have five baby chicks that are all just a couple hours apart from one another (they all hatched out in about 5 hours).

Now we are at day 13 and one of the chicks looks like it is significantly younger than the other ones. The normal five are starting to get wing and tail feathers and getting taller. The runt one (which my daughter is calling Lilliputian, or Lilly for short), looks like a newborn chick practically. It is small and hasnt developed feathers. But it is eating and drinking and vocal. I don't see anything wrong with it (no weird legs or chest that I can tell). However, it does a lot of pecking at the other chicks and it really is quite loud in comparison to the other ones. But not like it is in pain.

Does anyone have any ideas what is going on? I appreciate any advice/ideas/etc