Matt and I differ on the timetable for laying.
I want my females to lay no earlier than 8 months. My reason? If a pullet is good enough for me to keep as a breeder, I'm going to use her as such. I won't keep a bird that long if I know it's not going to be worth anything. So, by allowing the birds to wait until 8 months before laying eggs, the reproductive tract has a longer time to develop and also allows for a wider pubic bone spread. This means the eggs won't remain as small for nearly as long as a bird that lays at 6 months. So while others are waiting on their eggs to get some size, I can start setting eggs within a week or 2 of a pullet laying. I also won't be "wasting" eggs that could be used for hatching the next generation because they are too small to hatch from.
As for the males, I want the males to get to their maximum size. To me, having a male reaching sexual maturity fast means that they are simply busy breeding instead of building themselves. I prefer a male that starts breeding anywhere from 9 months or more. Course, they don't start looking ready for show until well after their 2nd year. That's more along the lines of feathering though and not the actual frame and mass.
Could someone breed for faster developing and maintain size? Absolutely! But let's just take a look at the production aspect of this. Leghorns (production) are a small breed and are meant to lay eggs earlier in life than other breeds. They can start laying eggs as early as 4.5 months in some strains. BUT, the size of the bird is small and they do not produce for a long amount of time.
Meat birds grow fast as well, usually in about 6 weeks. They are useless as breeders due to their bodies simply not being able to keep up their rate of growth for long.
I'm just saying that body size and production are inversely correlated. In order to get the size without ruining the integrity of the frame, you need to allow a bit of time for the body to shape up. Pushing it to fast, especially expecting a bird of that size to begin laying eggs at 6 months will (without good and intensive breeding practices) will result in a smaller bird.
It can be done, but be sure to not sacrifice the size for the early production.