One of my chicks isn’t growing??

I purchased a few Black Australorp/Brahma mix chicks on April 25th. They were about 5 days old when I brought them home. The first 4/5 days one of the chicks seemed extremely unstable on its legs/feet. Was panting heavy (I monitored the temp religiously - and none of my other chicks were showing signs of overheating and they’re much more feathered). I made sure to throw in some electrolytes into the water. It’s now behaving just fine, it eats, drinks, hangs with the big chicks just fine. They don’t pick on it. As of right now We’re a little over a week away from hitting the one month mark? and this little chick still isn’t growing?!? The other 4 have Developed beautiful feathering and are larger than the palm of my hand.. this one still looks as if it’s 3 days old and is just now getting tiny feathers but it’s size hasn’t changed.. I’m thinking possibly Mareks Disease?? Or maybe stunted growth? I really would like to get some input / opinions on what my next course of action should be??
I’ve attached photos of the smaller chick along with one of its hatch mates to give a visual as to the difference in size.. and these photos are about a week old.. so the others have grown even more since.. I’m slightly concerned, should I be? 😂
Thanks to all in advance!

I'd suggest there's something wrong internally with Little Chick. If it was an illness, I suspect it would have spread to the others, but if there's something internally wrong, such as part of the digestive tract twisted so food's not traveling through well, it may not be getting enough nutrition to grow.
When you see it eat, does its crop get full or is it mostly "pretending to eat"?
Is it just as lively and active as the others - just small?
One possibility is that it *was* ill, but overcame it enough not to die, but not enough to thrive. Do you have some naturally fermented foods you could try to get it to eat/drink in a small quantity in case it needs some "good bugs" in its gut.
In the past, when a chick's been "unsteady" it's been a slight stroke-like brain issue and we have had birds recover from that, but its food intake shouldn't have been affected.
Do you have a sensitive weigh scale? If so, please weigh it now and weigh it in a week if possible. With its mates growing so quickly, it may be growing very slowly and be hard to see when you're with them every day.

It doesn’t seem to eat as much as the others, but come feeding time it’s right there waiting and excited like the rest. It drinks plenty of water, and runs and plays With the others. It had acted strange the first week, with the unsteadiness and heavy breathing. Has since recovered from that. I will absolutely give the fermented foods a try! I greatly appreciate the tip, anything that may help..
I’ve not weighed it but it still feels as if it weighs virtually nothing. It’s managed to develop maybe 4/5 flight feathers but it’s hatch mates take me almost both hands to handle comfortably and This little thing is smaller than the palm of my hand. So developing maybe? But no growth..
I will look into a scale this week and give that a try!
Really appreciate the help!

This is a mystery! Usually, if a chick fails to thrive, it simply dies, and usually looks and acts sick. That may still happen.

So long as you make sure the others don't out-compete it for its share of feed, and you could consider giving it a little energy dense food in between the other feedings for a few days to see how it responds, you can wait a little longer.

There is a good chance that it will never catch up to its hatch-mates, so you need to start thinking about why you wanted chickens and whether there will be a place for an unproductive or under-performing staff member? It might be great for getting bugs out of small spots, but only lay banty-sized eggs if it lays at all, for example. If somehow it's digestive tract is damaged, the damage will likely be permanent, and it may or may not die spontaneously. Being a "cross", there's always the possibility that you just got some weird gene thing happening (we have human dwarfs after all) and you will have the equivalent of a banty. Banty's have a reputation for having "attitude" far beyond what their size would suggest, so I will continue to suggest you wait and see, but do some thinking in the meantime.