But, I really don't eat that much!

When and how did this weight creep up on me? One day I was a size 8 or 10, and just a few short years later it seems they are making clothes smaller than they used to!
How do we gain weight when we aren't really eating that much? The answer is, just as slowly as we lose it when we are on a calorie-restricted intake--a little bit at a time.
In fact, just because we noticed the weight all of a sudden doesn't mean we gained it all of a sudden. You might be gaining just one half pound a month. Not even noticeable for a year or so. In two years it has added up to a 12 pound total and you seem to have bumped up into the next clothing size "all of a sudden".
Even though you don't binge and gorge on food, and you may be actually trying to eat wisely most of the time, it doesn't take much to put on a few pounds a year.

Experts believe that it may come down to as little as 50 calories difference a day, and here's how: It takes approximately 3500 calories to equal one pound. Eat an extra 3500 calories and you gain a pound; exercise to burn an extra 3500 calories and you lose a pound. And this is true over a period of time. If you change your intake or exercise by 50 calories a day, it will take 70 days to change your weight by one pound. Certainly not a desirable rate if you are trying to lose weight! (Seventy days is more than two months.) But if you are taking in just 50 extra calories a day (say you've developed a new habit of popping a couple of chocolates in the afternoon, or switched to a large side of fries instead of a medium those 3 days a week you drive through the fast food place for lunch) you will end up gaining about five pounds during the period of a year. And what if you change jobs or houses and no longer walk a couple of flights of stairs every day or you now park closer so you're not walking as many steps each day? It could make the difference of 50 calories a day you are no longer burning--which translates to gaining five pounds this year.

Take a close look at your habits. You don't have to be eating huge amounts of food to be gaining weight over a long period of time. It might be a very small change you've made. The good news is, by making a very small change now, you can weigh five pounds less next year at this time! The slow weight loss is a trade-off for a small change--bigger changes mean faster weight loss. What can you change in your diet to take in 50 fewer calories a day? A bit less salad dressing or a lower fat variety? A smaller portion or a healthier snack? What about exercise--can you walk just ten more minutes a day or take the stairs instead of the elevator? These small changes will add up to a small weight loss month after month, until you're back into the size you like to be :)