Pet food quantity - large package or small package?

I buy my bird's main diet (Harrison's bird mash) in a 1 pound bag. It's only good for six weeks once it's opened, and the bag conveniently lasts almost exactly six weeks.

My cat eats mostly canned food, but he always has dry food available. I buy the 5.5 ounce cans, which last him two meals. I get the smallest bag I can of his dry food, because he doesn't go through it very fast. One 5 pound bag lasts him about 2 - 3 months.