When trying to lose weight the majority of people believe that a person needs to burn more calories than they take in and this is true to some extent but there are many factors that make this simple “calories in versus calories out” equation not work exactly like you would think.
For example if you take two people that are the same height, weight and gender there are situations where one of them is working out more frequently and eating fewer calories and not losing weight where the other is eating more and exercising less and losing more. But how can that be?
First you need to look at style of training. Longer Steady State Cardio is often thought of as one of the best ways to lose fat weight but infact it is one of the least effective. Steady state cardio burns calories while doing the exercise but once done expends very few recovery or “afterburn” calories. On top of this when you exercise for long bouts (over an hour, for steady state cardio, or 90 minutes, for a varied routine - though this is highly dependent on the individual) you increase your stress hormones which can cause your body to retain or gain fat weight even when exercising heavily. Strength Training and HIIT on the other hand are typically done for shorter periods of time which means you have fewer stress hormones released into your body so it does not actively try to work against fat loss. Strength Training and HIIT also have an “afterburn” effect which means that with less time exercising you can actually burn more total calories as compared to Steady State Cardio.
Second, how a person eats can have a huge effect on fat weight as well. Most people think that the fewer calories you take in the more fat weight you will lose and that also is not necessarily true. When your body starts to take in fewer calories than it uses you start to use your body's fat stores to make up the difference. When you start to pull from your fat stores too quickly (which is different for everyone) your body starts to intentionally slow its metabolism and goes into what's called “Starvation Mode”, where your body starts sending out hormones to slow your development of muscle tissue, your BMR (base number of calories your body burns doing nothing), and physical ability/energy during the day and during exercise.
How many times you eat a day can also have an effect. With the same number of calories coming in, someone who only eats two meals a day (or three with only one of them being large) will not see the same fat loss results as someone who spreads out those calories over 4-6 meals with similar calorie content. This is because when you only eat a few meals a day your body has to use your fat stores to get from one meal to the next and this sends a feed-back-loop to your body that you need your fat stores so don’t let them go. On the other hand if you eat frequent small meals your body rarely used fat stores (typically only when exercising) and so your body does not see the need to keep those stores any more if they are not being used as often so it is more likely to let them go.
If you take one person who is eating very few calories, only eating a few times a day, and only doing very large amounts of steady cardio, they could easily not lose or even gain weight due to the stress reaction that causes in the body. On the other side, if you take another person who is eating more calories, eating upwards of 6 times a day and doing Strength Training and HIIT for shorter bouts, they can easily maintain and even lose fat weight.
Related: My Experience: Cardio and Dieting Versus Fitness Blender and Clean Eating