You grab a banana, write it down in your food journal or track it on an app like MyFitnessPal, and assume it's just under 100 calories. These comparison photos from Instagram user _schneiderreich, however, show that portion size can make a huge difference when adding up your daily calories.
As you can see from the photo above, bananas vary so much in size, and if you guess the number of calories, you could be over or underestimating by almost 100! The same goes for peanut butter or other nut butters, handfuls of nuts or dried fruit, or any food, for that matter. Over the course of a week, if you make the same miscalculation on your daily banana (or whatever food it is), you'll consume almost 700 extra calories, which could be one of the reasons you're gaining weight. Likewise, if you underestimate, it could be the reason you're especially hungry or have low energy.
The solution? Before you can get good at eyeballing, take some time to measure out the food you eat in order to know exactly how many calories you're consuming. This definitely doesn't have to be a forever thing! Give it a few days or weeks and then you'll have a better grasp of what a handful of nuts or a piece of fruit really adds up to. It's just good to be aware!