People at buffets put more on their plates and eat more variety of items. How does this relate to the idea that diminishing marginal utility must be

People at buffets put more on their plates and eat more variety of items. How does this relate to the idea that diminishing marginal utility must be understood in context? Many people buy too much on their credit cards, even when they know they will be sorry when they get their credit card bill. Relate this fact to the idea of time in consistency.