Your body composition is the product of many factors.  Some of them you can't completely control.

Your Body Composition: Factors You Can Control

1) Type of activity you do. Different activities provide different stresses on different parts of the body, and as you would expect the body reacts differently to these stresses.  Aerobic exercise (jogging, walking, treadmill) has traditionally been thought of as the fat burning exercise, but there is more proof that being in your anaerobic zone not only burns fat, but increases your lean mass by building muscle and burning fat even faster.  You might not lose weight as fast, but you will get smaller, because fat takes up more space, pound for pound, than muscle.  A member of FAF JUST posted something about this, she has only lose two pounds over the past months, but reduced her body fat percentage from 27% to under 20%.  She gained several pounds of muscle during this period, so she looks leaner and meaner!

2) Amount of activity. Clearly, the more a person exercises the greater potential benefits in desirably altering body composition.  However, increasing activity too drastically without increasing food intake to balance energy can have a negative effect on body composition, as you can possibly start to eat up muscle.  In addition to overtraining, this will reduce your lean mass, even though the scale goes down, and reduce your power and endurance, which means in the long run you are moving backwards.  Find the balance that works for you, and eat appropriately!

3) Nutrition. Eating too much or too little can both negatively affect body composition.  As noted above, eating the appropriate amount for your activity level is essential.  If you are working out, you should aim to either match, or go slightly under your energy output with the food you take in.  For example, if you expend 2500 calories a day, consider eating somewhere between 2100-2500 a day.  Also note, failure to consume certain nutrients and vitamins can reduce your ability to metabolize fat and provide energy.  This is where we don't just "take away" bad foods in a diet plan, but must "add" good foods (vegetables, whole grains, lean protein, unsaturated fats) in their place.  The number one piece of advice I can give you is to eat more vegetables for this very reason.

Factors You Can't Control

1) Genetic Predisposition. This is everyone's bottom line, and no matter how hard you try, you can't change it.  We all inherit different body types.  It's just a fact, people with longer legs and shorter trunks tend to have lower body fat percentages.  IMPORTANT NOTE - regardless of body fat percentage, the inside of your body will still be altered negatively if you choose to eat poorly, even though you are at a healthy weight.  There are more and more stories about 20 and 30 year olds who are pre-diabetic because of their poor nutrition, even though they look completely lean and healthy on the outside!  Nonetheless, love your body. Don't use it as an excuse, or decide to not do anything about it if you want to improve, but love it nonetheless - it's the only one you get!

2) Age. Some say energy metabolism drops by 2% every decade after you are 30.  That means, if you are eating 2500 calories a day, you are 2% (50 calories) less efficient at burning fat every day once you hit 40.  If you don't adjust by burning 50 extra calories, or eating 50 fewer calories, over the course of a year you will gain (approximately) over 5 pounds.  Over 10 years, that would mean 50 pounds by the time you are 50.  This sucks! But still, it's a reality and you have to adjust by either increase activity or eat less.  That 50 calorie a day number is a good gauge, but if you are good at math, crunch the numbers for your daily diet and age.

3) Gender. All things equal, women have a higher body fat percentage than men.  There is nothing wrong with it, and nothing can be done to alter it  - it is just an evolutionary adjustment since women are the bearers of children.  This is not to say that their aren't many women who have lower body fat percentages than some men because they exercise more!

So what is your takeaway?  
1) Realize your genetic predisposition, age, and gender are key factors to your body composition.
2) Realize that you CAN make a difference to your body, but it's your decision! Don't compare to others too much or lose sight of accomplishable goals for YOU.
3) If you are in a rut, stop stressing about the scale, and check out the tape measure.  Inches in the long run tell you a lot more than pounds, unless you have a significant amount to lose.  Tell yourself you will work to improve performance by eating right and training hard, and the rest will come naturally.