Pesky carbs rear their hungry little heads.
NUTRITION Have you ever wondered why a small bag of French Fries doesn't seem like enough? Medium appears just as small and large gets you just a few more. Sometimes you want to tell the preparer to just fill a lunch bag with fries.
Well, you might as well stick to the small serving because no matter how many French Fries (or potato chips or bread) you eat (within reason), you are not likely to feel full. Satiation is a neurochemical response that occurs when your autonomic system signals that sufficient nutrients have been received. We assume it is a simple matter of filling a space but more complex processes are at work within us.
As a general rule, signals arising in the periphery that influence food intake and energy expenditure can be partitioned into two broad categories. The prototypical satiation signal is the duodenal peptide cholecystokinin (CCK), which is secreted in response to dietary lipid or protein and which activates receptors on local sensory nerves in the duodenum, sending a message to the brain via the vagus nerve that contributes to satiation.
The second category includes hormones such as insulin and leptin that are secreted in proportion to the amount of fat in the body. These “adiposity” hormones enter the brain by transport through the blood-brain barrier and interact with specific neuronal receptors primarily in the hypothalamus to affect energy balance. Satiation and adiposity signals interact with other factors in the hypothalamus and elsewhere in the brain to control appetite and body weight. 
French Fries are carbohydrates. Not all carbs are bad. Without a balance of protein and fat, you are likely to keep eating the carbs in search of protein. This is a particularly important concern for vegetarians since fries are typically one of the few vegetables on the menu at fast-food restaurants.
So here's a tip: try drinking an 8-ounce glass of water before the meal and eat a handful of nuts with the fries. Better yet, go somewhere and have a robust salad with nuts on top or some other form of protein. Vegetables are also carbs but they have sufficient fiber and other nutrients to slow the rate of digestion and curb hunger.