No matter what: if you eat below your BMR (basal metabolic rate), you WILL lose weight. The problem is that most people wildly miscalculate their BMR.

Your BMR is all the calories your body burns simply by existing. Throw a caloric deficit on top of that and you will lose weight.

Some people have ridiculously low BMR's that don't match their appetites. Some people with hormonal or thyroid issues might have a BMR of 1,000 calories. It sucks but it's true. Those people will be stuck overweight for life because it's pretty damn hard to eat under 1,000 calories for the rest of your life.

Other people have ridiculously high BMR's and really struggle to gain weight.

The key is finding your BMR through trial and error, and eating below that calorie rate or exercising yourself into a deficit.