How can the sizes of most fast food items lead to an unhealthy diet and/or
obesity?

Most of the fast-food contain a large amount of sugar, fats, and carbs and fewer minerals and vitamins.

This means that you are taking in large amounts of unhealthy calories in the shape of fast food which leads to weight gain.

While the portions have become large, the person will still eat the complete meal regardless of feeling full or not. This means that people are eating bigger portions of junk food leading to abnormal weight gain.

Fast food is also highly addictive, there is science behind why you always racing Mcdonald's fries.