The beak deformity is probably making it hard to eat, so the chick might be semi-starving. That could cause the small size and delayed growth. Or it could have other things wrong as well as the beak issue we can see.
Feel for the crop, to see how full it is. Compare with other chickens to get an idea of what is normal. Usually it should be empty first thing in the morning, partly full during the day, and really full at bedtime.
If the chick's crop is the right amount full each time, then I have no idea how to treat it.
But if the crop is not as full as it should be (after comparing with the healthy chicks), you could try a bowl of wet feed, which might be easier for the chick to eat. If it is able to fill its crop well this way, then a dish of wet food twice a day may be all you need to do.
If it still is not getting enough food, you could try tube feeding to fill the crop in the evening or maybe the morning. I've never personally done this, so I won't try to give instructions.
The other chickens might also bully that chick, especially as they grow older, so keep an eye on that too.
Personally, I would just cull the chick, but I know that's not the right choice for everyone, which is why I've tried to suggest other options as well.