The fact that they are the same age but there is such a prominent size difference worries me. Even if one is a male and the other is a female there shouldn't be such a large size difference. The smaller one looks like it has coccidiosis, especially by the way it's standing, which can stunt the chicks growth. You may want to look into treating the smaller one if it is not getting any larger.
As for their genders:
The larger one is a pullet. I can tell by the coloration of her feathers.
The smaller one is a cockerel. Note that there is less white in his feathers. Males typically develop slower, but I still don't think he should be that small. Does he always stand like that? Have notice any growth lately?