When updating to 1.21.2 all salmon in nearby chunks that were loaded in previous versions now will shrink to their smallest size
How to recreate this bug:
Create/Load a world in 1.21.1
Pickup a salmon with a bucket then place it back in the water so it doesn't despawn
Exit the game then reload that same world in 1.21.2
When you load that world the salmon will shrink down from their normal medium size to the smallest size
The first screenshot below is my salmon in my survival world after updating to 1.21.2 and the second screenshot is my salmon before updating to 1.21.2
I know they made it so that salmon can now spawn with different sizes in 1.21.2 but that was not suppose to effect salmon loaded in previous versions of the game
i hope your salmon are ok