Primarily the genetic disposition of the fish. Some anadramous populations show higher percentages of spending more time at sea:

Kenai kings are that way, one of the reasons they caome back so big.

OP steelhead have a tendancy to have a higher percentage of fish spending more time at sea vs. most Puget Sound streams, thus the fact that a great percentage of them are larger.

Not always sea time either, Skeena system steelhead usually spend another year in the rivers, allowing them to migrate out at a larger size and consequently they feed and grow faster in their sea time and come back larger even though at sea time is the same.

Genetics are important, and thus one of the reasons many of us are pro-release for wild steelhead ... their genetic diversity covers a broader spectrum than do salmon when it comes to life histories. Each one is important and if you only kill big fish, odds are they're gonna get smaller in succeeding generations ... studies on other fish species prove this!