This answer will really vary depending on the food and cooking method. In it would the over all bulk. IE cooking a 5lb whole chicken would take longer that 5lbs of chicken parts that are spread out in a pan. 1 lb of chicken parts would cook in about the same time as the 5lbs would. It would vary a bit depending on things like the pan being crowded, so lets say any amount of chicken parts which as spaced out the same would cook in about the same time. Although it might vary by a few min. because a larger amount of cold chicken would cause the oven to take longer to heat up to temp.
In my experience cutting something like a green bean casserole recipe in half would result in cooking if for about 3/4 of the time. doubling it would add maybe 1/4 or so. There again it goes to the over all bulk. The center of the cooked item would have to reach the right temp. The less of an item there is to get to the center, the shorter the cooking time.
AS for a crock pot while in general the same rule would apply as to bulk, a crock pot cooks at a lowe temp, so it is more forgiving. What I mean is if cutting a recipe in 1/2 and you cooked it the full time, it would likely be ok.
the of course there is the old vs new crock pot rules LOL
the newer pots cook at a bit higher temp (due to concerns about food poisoning) so basically if a recipe would be done in 8 hr on low in an old pot, it is usually done in 6 hrs on low in a new one
