Winters aren't always bigger than summers.. Their is a strain of B run steelhead that enter the Columbia that on average are monsters. They go all way back to the Snake, Clearwater, Salmon, etc. The biggest on record is 35lbs 1 oz. The state record for WA is 32 lbs 12 oz out of the EF of the Lewis in SW WA. But there has been plenty of rumors of winter fish in the mid 30's on the OP.

But to define the difference, it's just run timing and maturity level when they return to the river.
Keith