RE: Pet food quantity - large package or small package?
Well, the larger the package, the cheaper they actually are. So many people would buy huge bags of pet food. The expiration date on the package are usually meant for before the seal is removed. Once you started using it, it won't last nearly as long as the date on the label.
If you use look and smell to judge the freshness of the pet food, it isn't going to be accurate. Especially for the dry food. The more they expose to the air, the faster they decay.
One way I use to delay the decay is to use an even smaller bottle to get some food from the larger retail bottle, and I start to use the food in the smaller bottle. Until the smaller bottle is empty, I won't be opening the larger bottle again. It can keep the dry food in the larger bottle fresh a little longer by not open the bottle every day to expose to fresh oxygen.