If you’ve started counting calories, be wary when it comes to pouring that bowl of corn flakes. Compact cereals can cause you to accidentally overeat, reports a new study from Penn State University.

Researchers crushed standard wheat flakes into various sizes—full size and then 80 percent, 60 percent, and 40 percent the original size—and packed them into containers so that each weighed the same despite the differing volume. Once a week for a month, 40 adults were then given a random container for their breakfast and were told to pour and eat as much cereal as they pleased.

Even though the volume of the subjects’ portions in the bowl reduced when they ate the smaller flakes, they consumed 34 percent more calories compared to the standard size because they thought they were getting less cereal due to the broken pieces. “We have a notion of how much we should eat, but the physical properties, like the size and shape, of the individual pieces can often mislead us,” says Barbara Rolls, Ph.D., study coauthor. These participants were habitual cereal eaters and they reported that they thought they were consuming the same amount of calories each week. Instead, they were actually overeating.

If cereal is your kryptonite each morning, try choosing brands with bigger pieces—ones that are more like puffs, flakes, or wheat bites. “You’re going to get more satisfaction from foods that give you larger portions for the same number of calories,” says Rolls. This doesn’t mean you should avoid compact cereals like Cheerios—especially if they’re your favorite—just be aware of your inclination to chow down and cut your serving size down more than normal.