From my time fishing and also working as a habitat technician, and also from the history of other fishers whom I've talked to on both sides of the border, there has been a continuing decrease in the size and weight of the fish returning, at least onto the Canadian side of the border.
There are a number of different reasons for this, I think, but one of the potential ones that is really important to keep in mind is the targeting of large fish through large mesh nets for decades at a time, which scientifically has proven to be detrimental to the health of a run. As you mentioned, the larger the fish, the more fecund the fish, the better the returns. Also, a thing to understand, as well, as many people pointed out, is that this is a very long system. In the Yukon River itself, the salmon run is over 3,000 kilometres. That's the distance some of these fish go. You need large fish to make the return that far up the river. It just only really makes sense. The larger the fish, the more reserves it has, the more muscle mass it has to be able to make that long journey.
I hope that answered your question.