how long until s bantam cochin is full grown

[chickenreyna]

Minus feathering out what age would you consider your bantam cochin is almost done growing or looks practically same size as am adult?

 

[chickenreyna]

For example i have a barred rock and ee that were born approx first of September who in beginning of December besides not being as heavy or fluffy looking were about same height as my adult birds i have