Grapes are not fattening if they are eaten according to normal serving sizes. According to WebMD, one cup of grapes has around 100 calories, which makes them a healthy snack. The same amount of grapes has less than one gram of fat.
One pound of fat contains 3,500 calories, which means that a person would have to eat around 35 cups of grapes beyond his or her regular calorie intake to gain an extra pound from the delicious fruit. This amounts to several pounds of grapes, which is much more than even a voracious snacker is likely to eat in a regular day, week or month. Grapes are especially unlikely to cause weight gain if a person replaces a less healthy snack, such as a milkshake or a chocolate bar, with a cup of grapes every day. This reduces the amount of calories and fat consumed, and the fiber in grapes creates a feeling of fullness that tends to stop further snacking. In addition to being a healthy alternative snack, grapes, and especially grape seeds, contain antioxidants, which some studies show are important in fighting cancer. As with any snack, grapes eaten in excess or with condiments, such as sugar or syrup, can increase a person's daily calorie intake, causing an increase in weight.