A chick that is very small and that is lethargic is likely a "failure-to-thrive" chick. These chicks hatch with genetic flaws such as undeveloped internal organs. They do not process the nutrients from food, so they don't grow. They are usually cold for the same reason. They get constipated very easily and they are also prone to pasty butt.
They have everything going against them. What you can do is give Poultry Nutri-drench, a potent vitamin supplement that is designed to go right into the blood stream, by-passing a possibly undeveloped digestive system.
Watch for the chick to poop. If it squats and strains and peeps loudly as it does this, it's constipated and needs treatment. If it has a clogged vent, you need to clean it.
It helps to give a chick like this minced boiled egg or crumbled tofu since these are easier to digest than crumbles.
Sometimes these chicks manage to make it, but more often they die despite everything we do.
