Brahma.. being a larger breed can have a slower growth and maturity rate than some others. Also individual genetics will vary and you can see these anomalies even among the same breed from the same parents.
Also feed and temperature can play a role in how much energy is used for growing...
I agree that's a stark difference in sizes. Maybe age is inaccurate or bantam is another possibility.
As long as behavior is normal I would not be alarmed. That chicks feathers look like heck... was it being raised by a broody or in crowded conditions? Have you checked it for parasites? It could be caused if they were using too low of protein feed since feathers are made from 90% protein and its' amino acids. So many possibilities!
Are you using a starter or flock raiser feed with not less that 20% protein? Are you offering any treats or supplements?
Anyways, pretty babies! Rest well.