What Kathy said... I believe the OP is seeing this sibling size/type discrepancy in action in these chooks.
The photos don't show any obvious problems. There isn't a drastic size difference, certainly I don't see the typical signs of stunting or thyroid/endocrine disorders.
I notice all the...
It's fairly normal to have runts among chooks as well. Normally they're culled.
Sometimes it's due to harmless genetics, sometimes due to harmful ones. Could be genes affecting the legs, or the pituitary or thyroid; the causes are legion, seriously. Could be a scarred bowel preventing her...