The Relationship between Muscle, Weight & Size

By: Ben Longley

It is very important to be able to discern between losing weight, and losing body fat.

After spending the better part of 2 weeks in hospital recently, not sleeping more than about 4 hours per night, and not eating nearly as much as I usually do, (I went for 2 whole 24 hour periods of complete fasting!), with no exercise, I lost about 5kg.

Now this is NOT good weight loss by any stretch of the imagination, this 5kg was probably a bit of of body fat with the majority being muscle tissue.

Had I kept living a similar lifestyle (of starving myself, not getting enough sleep, and no exercise) I would soon end up with a much higher body fat percentage and would be quite out of shape in no time.

So whilst 5kg of weight loss might sound great to a lot of you, there is a big difference in HOW you lose 5kg and what that 5kg is actually composed of.

Weight loss on the scales CAN be a bad thing, and it certainly doesn't mean you will definitely look better, or be healthier just because you've lost some kg's.

For example here are some quick ways to drop some weight quickly if you really want to:

  • Amputate an arm or leg

  • Osteoporosis

  • Dehydration

  • Get the flu

  • Shave off all your hair

  • Lobotomy

So really what we're after is a 5kg loss of 100% BODY FAT; this is the ideal outcome. To do that we need to maintain muscle tissue, and ideally gain muscle tissue.

It's important for females in particular to understand that this is how you really change how your body looks (in terms of muscle tone and shape), and in doing so will increase your ability to burn body fat and improve body composition over the long run... (So basically how you will look in 2, 5, 7, 10 years from now).

As you get older, you will lose muscle tissue if you don't pro-actively maintain or increase it. So your daily eating patterns now won't be tolerated as well in 2, or 5, or 10 years from now. You can learn this the hard way (by looking in the mirror in 10 years and realising your body has gone really down hill) or you can take action now and be in control.

Here is a familiar scenario: In January, Sally and John agree to lose weight together. Sally watches what she eats, counts every calorie, and spends hours on the treadmill every day. After a month, she's down by 2kg.

Meanwhile, John decides to drink less soft drink and manages to cut down to one can a week from his usual four. He gets to the gym when he can - maybe three times a week - but half the time, he ends up cutting his workout short. One month of this, and he is 5kg lighter!

What the heck? Why does this happen? (I can hear all females gritting their teeth from here!)

There are many physiological reasons, but the difference in their muscle mass is one of the biggies. The extra muscle tissue allows John to burn a lot more calories at rest and during exercise.

Let's compare two women this time - Sally and Mary both have the same amount of fat, but Mary has an extra 7kg of muscle.

If, for one year, Sally did exactly what Mary did to MAINTAIN her weight - go to work, sleep, play netball, whatever - Sally would actually gain 5-10kg of fat. Due to the differences in their resting muscle mass, Sally just isn't expending the same amount of calories day to day, and since Mary has more muscle and weighs more overall, despite having the same amount of fat, she actually has a lower percentage of body fat.

Weight Versus Size
Deciding on an arbitrary weight that you think you should be, really isn't an indication of anything. I could line up 10 females all with the exact same weight & height on paper, but they could all have completely different looking bodies, fit into different dress sizes and have BIG differences in body fat percentage.

Since muscle is more dense than fat, 1 kg of muscle will take up less space than 1 kg of fat. Muscle density is 1.06 kg per litre of space and fat density is 0.9196 kg per litre of space.

If you gained 10kg of muscle at the same time you lost 10kg of fat, you would be smaller, about 1.4 liters smaller. On the scale you would weigh the same but your pants would be looser and your metabolism would have increased so now you're burning more calories even when resting, allowing you to tolerate eating more food to stay at that same weight.

Let's say you and your friend decide to start two different weight loss programs at the same time. After 6 months, you've lost 10 kg by working out, eliminating alot of processed foods & replacing with good healthy foods, while your friend has lost 11 kg by lying in bed drinking coffee and smoking.

Your 10kg scale weight loss might equal a 10 kg muscle gain with a 20 kg fat loss. If so, you'd be 12.3 liters smaller.

On the scale, it would look like your friend who lost 11 kg (9 kg of muscle and 2 kg of fat) was doing better, but in fact, she'd only be 10.7 liters smaller, making her 1.6 liters bigger than you and not looking nearly as good for a similar weight on the scales. Ha!

Meanwhile, going forward, who will maintain her new weight more effectively? It sure won't be your friend.

Of course, this is an oversimplification, because muscle and fat are not the only things at play. But the message is the same - losing weight is very different from losing fat.

About Ben:
Ben Longley is the owner of The Fit Stop, a personal training & group training facility in St Kilda East, Melbourne. The Fit Stop specialises in helping local residents lose weight, tone up, and get in the best shape of their lives using only the most tried and tested, proven training methods and nutrition advice.

Ben has a Health Science background at university where he majored in human anatomy and physiology and has been a personal trainer for over 10 years, having worked with literally hundreds of clients over this time in 1-on-1 training, small group training and large bootcamps. Ben has a keen passion for all things to do with fat loss, strength & conditioning and nutrition and is a big believer in constantly learning and improving, and still continues with his ongoing education on a daily basis.

Ben writes his own health & fitness blog over at and you can find out more about his services by clicking the link below The Fit Stop - Personal training St Kilda East


May 23 2013 1:21PM
Great article Ben! I get really frustrated with my girlfriends who constantly talk about their weight in kilos and how they need to lose 2kgs or 3kgs or whatever. I have exercised frequently my whole life and rarely weigh myself as I have always found it a confusing indicator. I am slim by most standards but am also strong and fit and when I do tell people my weight in kilos they are often shocked as they assume I will weigh less. I know when I look and feel good - it is when I have more muscle and I feel strong. As long as when I squeeze my bum it is firm rather than flabby and soft, I know I am looking ok! My clothes fit better because my body is more toned and there's less fat to squeeze around and my posture is better which always helps too. Anyway, I do wish more women (in particular) would focus on fitness and strength than their BMI or weight.
Comment by: Sarah

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Sarah on The Relationship between Muscle, Weight & Size :
Great article Ben! I get really frustrated with my girl... »
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