Mindless Eating by Brian Wansink, Ph.D who heads the Cornell University Food and Brand Lab. Here is some data that he presents in his book:
On average normal weight people think they eat 20% less than they really do. Obese individuals typically underestimate how much they eat by 30-40% with some as high as 50%.
Your first step is to determine how many calories you actually consume in a day. Most people have no idea what that number is. Your best bet is to keep a food journal for 3-5 days (with a weekend added in there). More to come..