We eat several hundred extra calories more a day on average than we did 30 years ago. Where do the extra calories come from? In large part, bigger portion sizes.
If you want to know how much you're eating, try this experiment. Pour a typical bowl of breakfast cereal, then measure the amount in your bowl. How does it compare that to the serving size on the label? Do the same for everything you eat, and you'll see exactly where you can make changes.