How does this work? I have way more muscle than my wife, but she can eat way more than me without gaining weight. It can't be just muscle mass?

How many calories does extra muscle mass burn daily?

There could be a lot of factors: your body size, muscle to fat ratio and body comp, total exercise in a day, the actual caloric amount of the food you are eating (you can eat a lot of low calorie food compared to a single high calorie item for instance), mitochondrial function in your muscles and your ability to clear lactate from your bloodstream. Controlling for all of the variables is actually very difficult.

Most people that eat a lot and don't seem to gain weight are simply not eating that much calorically or exercising a lot. When a more sedentary lifestyle kicks in they generally begin to align more with the general population.

It's definitely not just about muscle mass. But you should compare yourself to yourself only when it comes to self-improvement, since you can't borrow someone else's metabolism. Having more muscle mass is better than not having it for both health and appearance.

When we're talking about slowly gaining weight over time as the OP did, even if muscle only burns a little more energy than fat that could be enough to tip the balance.

Perhaps the difference comes down to body type (Ectomorph, Endomorph, or a Mesomorph)?

https://www.gq.com/story/body-types-explained-ectomorph-endo...