Hey AC,
I would be lying to you if I said I knew for sure.
Every healthy population of salmonids has a wide range of size and run timing variations. The strength of a particular stock of fish is rooted in the resiliency borne of genetic variability.
In plain english, if all individuals in a particular stock of salmon exited the river as juveniles in one mass, couldn't they get wiped out by a flock of cormorants and/or terns?
If the same stock fed in the same place in the ocean, for the same period of time and returned in one school, couldn't they get harvested by one purse seiner or gillnet opening?
Mother Nature has "hedged the bet" by spreading out populations spatially and temporally, eliminating the possibility of a population all suffering the same, unfortunate fate.
The larger individuals in a stock of coho may have an extra year of fresh water growth but more likely spent a bit more time on the oceanic pasture or found a particularly rich area in which to feed and grow quickly.
I've been fortunate enough to find 20 pound class coho in the Skagit, Snohomish and Cowlitz systems and undoubtedly there are more systems that produce huge silvers!
If we ask nicely, perhaps we might get Smalma to "weigh in" on this issue.."Weigh in"...get it???