With regards to North American grizzly bears, individuals among the extinct California variety were documented to be as large as coastal Alaskan bears. This might have been for the same reason: salmon. Of course, the Alaskan bears didn’t start to see the kind of human hunting pressure that the California bears saw as early as the late 1700’s. Mexicans were capturing bears to fight bulls long before Americans started arriving in search of gold around 1850.
Great Plains wolves & grizzlies, preying on giant bison, were also reported to be huge before they and their prey were extirpated by Americans.
The food/size thing for sasquatches might be the same. West Coast sasquatches, benefitting from large, regular runs of protein rich salmon, were likely larger than, say, sasquatches from the Texas/Oklahoma/Arkansas/Louisiana corner. But as we destroyed the salmon and simply overran the coast, the sasquatches (like the bears and wolves before them), have died off or moved north.
(Note: I think sasquatches are behaviorally more like pumas than bears/wolves with regard to human pressure; more secretive, quiet, and nocturnal. Both brown bears and wolves are less tolerant of intense human pressure, but recent Alaskan studies are proving that given the huge spaces needed nearby, both species can and will quietly infiltrate even dense cities of humans with remarkable secrecy. I believe sasquatches do likewise...........)