Fingerlings arrive at the cages weighing a few grams and are graded as they go in. Uniformity at stocking is what produces a uniform harvest: a cage that starts mixed stays mixed all the way to the processing line.
Stocking density is kept low on purpose. Fewer fish per cubic metre lets stronger currents move through the cage, and that is what gives the flesh its firmness.
From there it is time and feed. Our species run different cycles, so cages are stocked in batches through the season rather than all at once.