Our hatcheries sort the trout by size depending upon where they will be stocked. TMAs, especially the Farmington, get the biggest available trout from the rearing ponds excluding breeders. Trophy Trout streams, like the Pomperaug River, get the next priority for the biggest trout. Other larger rivers, where the limit is five, will get mediums to larger trout. Smaller major trout steams with high fishing pressure in high population areas like the Norwalk River get mostly mediums with some larger trout and get stocked multiple times in the spring.
Smaller brooks stocked once a year pre-season get small to medium fish.
One of the problems with raising trout is they all don't grow the same just like with any species. If you raise 100,000 rainbows maybe 20,000 of those rainbows will be "runts" and no matter how long you keep them in the rearing ponds and no matter how much food you force down their throats, they simply won't get to 16 inches in two years so it is a waste to raise those fish. You sort them out and stock them early in the smaller streams for trout season and then sort out the largest fish in that year class for raising to 16 inches over two years because those fish represent your optimum mix of food, time and growth. For instance, when the county conservation commissions have their spring plant and tree sales they often offer rainbow trout for sale from a hatchery who pulls your permit and delivers the rainbows to your pond. This is a bad purchase because these rainbows are all "runts" sorted out of their year class and pawned off on an unsuspecting pond owners and are basically nothing but high priced bass chow.
So you simply can't get all big fish out of a year class. Plus trout like brookies don't grow as fast as rainbows so they tend to be smaller or take a lot more effort to raise. A nice 14 inch brookie is at least two years old and required higher quality colder water to raise.
I used to raise trout and had both brookies and rainbows. The brookies just could not compete with the rainbows for trout chow. The rainbows would go wild but the brookies would just swim around taking a pellet here and a pellet there whereas the rainbows were just eating as fast as they could swallow. It would be like putting a bowl of dog food down in front of a big lab and a little lap dog. The lab would eat everything in one gulp and the lap dog would get nothing and be lucky to not get eaten himself.
Browns can also be fussy. From a hatchery standpoint rainbows are the best return on investment from weight gained from food expended, time and labor. If you go into a fish market do you ever see browns and brookies? It is always farm-raised rainbows, steelhead or salmon. Just some observations.