Size of water perhaps. If you only have a 100 fish in a 20 mile loch then less than 20, perhaps even 10 fully grown ferox in Calder? It would be more beneficial to the fish from a breeding perspective to be more numerous at a population of 20 rather than 10 or less, so downsizing would be a survival plan.
Again I am in wild guess territory?
I do know someone who claims to have had a 'we are gonna need a bigger boat moment' when something jaws like cruised past one summers evening.
Now, does that mean these larger trout are actually salmon, and vice versa?