system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How does a food company determine what a serving size is?

For example, Kraft peanut butter's serving size is 15g. Why not 10 grams or 20 grams?
gpt
They're required to put the nutrition on their labels, but after that it's fair game.  They typically want to put a nice number on there for calories but will disguise by dividing it and calling it a "serving size".

People will see "100 calories" and like that part, even if there are 10 servings and the whole thing is about 1,000 calories.