Assume you consume 100 calories a day more than you need. Assume all of those calories are converted to fat.
9 calories = 1 g of fat, so 100 calories = 11.11 g of fat.
11.11 g = 0.024 pounds per day.
0.024 x 365 = 8.9 pounds a year.
So simply eating only 100 calories a day more than a person needs will cause them to put on nearly 9 pounds a year. Downing a "Super Big Gulp" a day could easily double or triple that amount.
I think that's one reason why it is so easy for so many people to gain weight - it's easy to lose track of a few hundred calories a day.
Cheers,
Scott.