Presumably the optimal size for survival when humans aren't applying pressure via fishing is bigger. Perhaps "ruining" is hyperbolic, but this is making the fish less fit for their environment.

In the sense that the gene pool is being unintentionally manipulated by humans, when the original goal was to try and leave it undisturbed to an extent.

if you're interested in maintenance of the fishery, size reduction is a primary indicator of failure. so yes

Yes. It is.

It's better if you want to eat them lol

> Is being bigger Platonically better than being smaller?

Maybe, most animals evolve towards larger sizes. Except on smallish islands, where they tend to become smaller.