I assure you it is much more complicated than you suggest. To the point that is an active point of research and debate. It has been shown that a person’s base metabolic rate is not a fixed number which can be affected by things such as caloric intake, types of calories and exercise(independent of the calories consumed during exercise). A simple reduction in calories intake will often result in a crash in the metabolic rate and then the sequential rapid gain weight when normal calorie intake is resumed.

In addition to this it has been shown that people are getting fatter on less food than they did 40 years ago. This is vastly more complicated than a Newtonian model of the body would suggest. Yes, diet and exercise can help reduce being overweight. But to merely suggest somebody should quote eat less is ineffective and is not shown to hold up in studies. Oh and sorry about the pay wall.

https://academic.oup.com/fampra/article/16/2/196/480196?login=false

https://www.theatlantic.com/health/archive/2015/09/why-it-was-easier-to-be-skinny-in-the-1980s/407974/