If you’ve ever been to a group fitness class where everyone’s heart rates and estimated calorie burns are displayed on a screen, you know that these statistics vary greatly from person to person. You’ve probably also noticed that, generally, men tend to burn more calories than women. But have you ever wondered why different people burn calories at such different rates, even during the same workout?

The truth is that metabolism – an umbrella term for all the processes in your body that break down nutrients for energy, fuel growth, and more – is far from simple. “There is a constant ebb and flow of reactions that build or repair our body (anabolism) and reactions that break down food and energy stores for fuel (catabolism),” says Anya Rosen, a functional medicine practitioner. Various factors play into how fast or slow you’re burning calories at any given time. Here are the five that experts say have the greatest impact on how many calories you burn while working out.

1. Body Weight
“Generally, the more you weigh, the more calories you’ll burn per session,” says Kyle Gonzalez, the head of content for a supplement company. “Calories are just a measure of energy, so the more you weigh, the more energy it takes to move your body.” Put differently, of two people with different weights, the one who weighs more will burn more calories, because they have a greater energy expenditure when moving.

People with larger bodies also tend to have larger internal organs which is a significant factor in how many calories are burned during exercise and at rest, because these organs and their processes require energy. This is one of many reasons that weight loss is so complicated – your body burns fewer calories as your weight decreases, which can lead to a weight loss plateau or even regaining weight.

2. Muscle Mass
Here’s where things get a little bit trickier. Someone with more muscle mass will burn more calories than someone else who weighs the same but has less muscle. “Muscle tissue burns more calories than fat tissue,” says Jenaed Brodell, a sports nutritionist.

During exercise, having more muscle mass will increase your total calorie burn, because your body needs to produce more energy to support the increased rate at which your muscles are contracting. Long story short, if you want to enhance your calorie burn, consider stepping up your strength-training game.

3. Age
“As we age, we tend to lose muscle mass,” Brodell says. “After age 30, you begin to lose as much as 3 to 5 percent of your muscle mass per decade.” The reasons for this aren’t perfectly understood, but one review explains that it’s likely because your body becomes more resistant to hormones that promote the protein synthesis that’s key to muscle maintenance. This loss of muscle mass lowers your metabolic rate – the speed at which you burn calories – at rest and during exercise.

While you can’t stop your body from aging, you can preserve or even increase your muscle mass with regular strength training, Gonzalez says. “Strength training can help you increase your resting metabolic rate, which helps you burn more calories at rest over time.”

4. Fitness Level
The more you do a certain type of workout, the easier it seems. That’s not in your head – your body actually does adapt to do things more easily over time, Gonzalez explains. Overall, this is a good thing. It means that you can run faster or for longer with practice, and your muscles will be able to lift heavier weights with proper training.

But it also affects your calorie burn. “As your body adapts to training, you will burn less calories with the same workouts,” Gonzalez says. “From your lungs to your muscles to your heart to your brain, your body becomes more efficient as you become more fit.” That’s why a newbie might burn significantly more calories than someone who’s been doing the same workout for years.

5. Training Intensity
It’s also possible that two people doing the same workout are burning a different number of calories because they’re not actually doing the same workout. Brodell says that someone exercising at a high intensity, meaning you’re breathing heavily and can’t carry on a conversation, can burn twice as many calories in the same amount of time as someone exercising at a low intensity.

The bottom line is that exercise has countless benefits beyond just burning calories, so the most important thing is to find types of movement that are enjoyable and feel sustainable. “The type of exercise that is better for a person ultimately depends on that person’s goals, physical fitness, and capabilities,” Brodell says.