One slice of plain cheese pizza has approximately 400 calories. So eating even two or three slices of pizza will add 800 to 1,200 calories in your diet. This will be higher when you add processed toppings like pepperoni.
Since a person should be consuming 2,000 calories on average in a day, these few slices of pizza will take up between 40 to 60% (or even more!) of your daily calories intake. When you also add calories from your other meals of the day, it is easier to do overboard. Over-consuming calories regularly will quickly increase your weight.