Pet food quantity - large package or small package?

I used to buy smaller quantities of fish food for my koi, because as Ram stated, the quality of the feed tends to deteriorate over time once the pack has been opened and exposed to air. I used to live in Southeast Asia and this was pretty much obviously due to the high humidity where I lived (84-85% on normal days and sometimes even higher).
When I lived in Madrid, I would buy large amounts of food for my goldfish and store it for a long time. Once I left the pack open for 3 and a half weeks as an experiment during the even drier Spanish winter. There was no change in the smell, color and texture at all.
This is something that you could never do in Thailand, India or Singapore. The humidity just gets right in and encourages oxidation and all kinds of nasty molds and fungi to spoil everything.