I they are "extra roos" then it doesn't really matter how much meat they have on them. You aren't raising them to meet weight cut-offs for commercial meat birds. If they are smaller than what your family is used to, serve two at a meal instead of one.
Out of curriosity what do you mean by meat bird food? Are you dairy fattening them in a crate? If so, no you won't see as large of weight gains from a laying breed and you would a meat breed. Meat birds have narrow hips and wide breast. The wide breast is optimal for gaining weight during the finishing process. Laying breeds have narrow breast and wide hips. The wide hips are optimal for prolific laying, but the reduced breast area means they won't gain as much weight during fattening. The meat is just as good though. I actually like the Legbar meat better than all the "meat birds" that I have tasted.
Note: Poussin (aka spring chicken, springs, or game hen) is baby chicken harvested at around 1 lbs. It is a delicacy and proves that no chicken is ever too young to butcher or ever has too little meat to eat.