To increase uniformity in rearing it is a good practice to grade the birds in weight groups. This gives the opportunity to adjust the feed for bigger and smaller birds and get the total group more uniform.
However, we have to realise that the reason those birds are smaller (or bigger) is because of a difference in dominancy, induced by the competition between birds. A bird that has not been able to compete and will therefore not get its fair share of feed, will of course stay smaller. If we put this bird in a separate together with other small birds, it has more chance to get its share of feed and will catch up in weight. But we have to realise that with it we do not automatically also increase the dominancy and with it the ability of the bird to compete with the bigger birds in the flock.
That means that when we place the birds back in the normal flock after the period that they needed to catch up, often they will fall back again, simply because they still are not able to compete with the bigger birds. This of course will be more the case when the stocking density is high and the feeder space is low.
One of the ways to overcome this is to not wait too long with grading. When the grading is done early, birds will not have established their typical individual dominant and sub-dominant behavior that much. It is typically observed that when we grade the flock early (before 4 weeks of age) the number of big birds in the flock stays smaller later in life.