Having a longer lifespan might explain the higher nurture/nature ratio. Cats and dogs don't live long enough to develop complex background stories that require abstract language to be transmitted to their children.

(a) Whales have big brains, whales use them for complex behaviour.

(b) Brain size is relatively unrelated to complex behaviour.

Kinda "a dog has four legs, a dog uses its legs to run" vs "number of legs unrelated to running ability, my kitchen table isn't running anywhere".