You can't overeat on leafy vegetables. It is just not possible. They do not have any calories. With some vegetables, the energy you invest in chopping and eating them is more than the energy you get from them!
Even with root vegetables like carrots, you can't add a significant amount of calories to your diet by eating them, because you're full before you get the normal calories for a meal.
Adding bananas to your normal diet is probably not going to make you lose weight, but with vegetable, I really don't see the problem.
Just eat the vegetables first, then eat pizza until you are not hungry anymore, and put the rest of the pizza in the fridge.
If poster means that people are adding salad and veg and not cutting down the other stuff then I kind of get it.
To be honest, I doubt that is possible, even.
Unless you eat the pizza first and then force yourself to swallow the vegetables ... veggies take up space in your stomach that means that less pizza fits in. You can't just double the volume of what you eat, surely?