Pet food quantity - large package or small package?

Pet food don't last forever, not even dried pet food.
Think about beef jerky, or cereal. Sure they can last for a while if the package is still sealed. Once the package is opened, they can't stay good indefinitely, right?

As a fish keeper, I have come to learn that it is the best to buy smaller bottles of fish food than the big giant ones. Even though the dry fish food might stay good for months or even longer after the seal is opened, the nutritious value decay over time. Especially the vitamins, from the moment you opened the seal you get less and less of them.

It should be the same for all other pet food.
Do you buy smaller packages of pet food for the freshness?
Or do you buy large packages for the bulk discount?