Sometimes you just have a runt. I had a black silkie cockerel this year who was absolutely tiny compared to his hatch mates, at 10 weeks he was about the size of maybe a 7 week old. He ate plenty, just didn't grow as much.
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Not the best photo, but it does show the size difference okay. I re-homed him to a breeder who was looking to breed tiny Silkies. He was healthy, very active, just small.
So the one your worried about hangs out under the heat a lot more than the rest? How close is the food and water? Maybe moving it closer or putting a little lid with mash in it next to it will encourage it to eat more?
They do sleep a ton at this age as well, they generally run out eat and run back to the heat and sleep in intervals all day long.