When calculating how many calories you need in a given day, it is important to remember that the calorie intake for men will be different than the amount of calories women need. There are a few reasons for this, but the most important is weight.
The average woman weighs quite a bit less than the average man. It is an easily observed difference between men and women that has some important consequences with regards to calorie needs. A 125 lb woman that gets an average amount of exercise throughout the day will need about 2,000 calories. A 175 lb man who gets an average amount of movement in throughout the day, on the other hand, needs about 2,800 calories to see him through the day. The 800 calories of difference between the two can be mostly attributed to the 50 pounds of weight difference.
The different caloric needs can best be illustrated with an example. If you have ever been backpacking, then you probably knows what it feels like to carry around a large amount of extra weight for a day. Some backpacks can get quite heavy, over 50 pounds in some cases. At the end of a long day of carrying an extra 50 pounds or more, your legs are tired and generally, you feel exhausted. Getting that backpack off is a relief.
Now imagine carrying around that 50 extra pounds of weight every day, for the entire day. Imagine how many calories you would burn. That is where the difference of 800 calories per day comes from, that extra effort that it takes to carry around that additional 50 pounds. That weight is gained gradually, and isn't added all at once, like with a backpack, but the two situations are very similar.