I think it's really common for people to look at big items and ignore or discount the small things. We think of our workout, but not the smaller bits of exercise we get throughout the day. We think of our meals, but not the calories consumed through small snacks.
I thought of this the other day when I missed my workout. I didn't get my 30-45 minutes of cross-training in that day. However, I did walk from the train to work and back. Each way was about 3/4 mile. I have also switched my desk at work from sitting to standing. That change is burning about 116 more calories per day than sitting. I've also been doing my physical therapy exercises, but not counting any of that time as "working out."
I think the same thing happens to the calories we consume. We pay attention to the meals, but not the small bites. It's really easy for those unaccounted bites to add up to enough calories that your body will show them, even if you didn't think of them while eating or drinking. When discussing obesity, people discuss how portion sizes have increased over the years, they don't talk about mindless small bites. We consider the calories in the french fries, but not the ketchup.
I think this is a huge part of the childhood obesity epidemic. Children are constantly being hydrated and fed with calorie-laden items. A constant stream of calories that never amount to enough to actually satiate. That means that they can keep eating and drinking well past the point where they're reached their caloric needs. This leads to weight gain. It's not sudden, it's very gradual and then suddenly it's a health issue. The same thing happens to us as adults.
I'm not suggesting we all skip our workouts in favor of small burst of activity and also skip all snacks and only eat large meals. What we should all do is consider the whole picture of our lives, not just the large items. Give yourself credit for walking all over Target for thirty minutes. Track the calories from that extra bite you took after you were basically done with dinner.