This is why we have legally-binding ages which signify when someone is a minor or an adult. Physical differences can be so different from one person to the next. Thinking that taller or more built people are more "adult" than others is already a large-scale bias, but let's not allow it into the legal system.
There are already studies showing that young black males are viewed as older and less innocent.
It is most certainly NOT justification for such things. That said, from the standpoint of curiosity alone, I wonder how the prison population compares to the general population in height and weight? I suppose weight is more informed by your surroundings while height is something you are simply predisposed to. Anyone know how the comparison shakes out?