This difference has nothing to do with the fact that a woman's body functions in some ways differently than a man's. Women's muscles and organs burn calories and fat at the same rate per pound as a man's muscles and organs.

The real reason is much simpler.

It's that men are generally larger than women and have more muscle as a percentage of their total weight.

More muscle burns more calories.

This means that if a woman and a man both "overeat" the same number of calories at a meal, the man will probably burn more of the calories and store fewer of them than the woman. Similarly, if they are both on a diet and "undereat" the same number of calories, the man's body will probably pull more fat calories out of storage per day than the woman's body.

How big is this difference? It's not very big.  (But it adds up over time.)

The following example from the table below is for a man (5'9") and a woman (5'7") both 30 years old, with an "office worker" activity level  (i.e. very light),  no additional exercise, and a BMI of 24  (meaning they both have the same slightly unhealthy degree of "fatness").

The man's body will burn 374 more calories per day than the woman's body and will therefore gain weight more slowly or lose it more rapidly -- assuming they both eat the same number of Calories per day.

However, men don't have all the advantages.

Women can usually carry a higher percentage of their weight as fat than men can before they begin to look "overweight" to most other people.

Although there is a high degree of subjectivity in this, women are generally regarded as appearing "healthy and feminine" up to about 20% of their weight as fat.

Men begin to be viewed as "sloppy" and "out of shape" when they carry more than about 15% of their weight as fat.