That illustration above is a perfect example. The picture on the right is roughly the equivalent of what you would have received in your fast food bag in the 1960-70s. Fast forward 40 years, and the average trip to McDonald's has most if not all your daily calories in a single trip. Without getting into further specifics about all the chemicals and crap that are put into your average fast food meal, eating one of these double and triple portion meals a week as well as the other couples meals you're probably consuming in a day is going to add significant weight to your body year over year.
I use fast food as an example, but portions have blown up everywhere, and the only sure fire way to make sure you're not overdoing it is to be imparted with the knowledge of both what you're putting into your body and what qualifies as a serving. I've slightly covered this issue before in my Food Labels 101 post, which I recommend you read when you have a chance.
As it pertains to portion control, knowledge is power. Read the labels on things you buy. Almost everything will you tell you a portion size in grams or ounces. If you have access to a food scale, weighing your food out may seem like a big task, but it's a guaranteed way to know how much you're putting in your body. As most produce and fruit is not labeled, though, the FDA recommends measuring in cups. For example, the average adult woman should have between 2 and 3 cups of vegetables a day. Having a measuring cup around your house is a very easy way to make sure you're staying on track. If you don't have access to measuring equipment, use this chart to relate your foods to common items. Click on the picture to view or download the full size PDF.