The reason why the muscle circumference of middle-aged men is larger than that of young men who exercise regularly may be due to the following reasons:

1. Hormone levels: middle-aged men The hormone levels of men are relatively stable, while in young men, hormone levels may fluctuate due to growth and development and other processes. Stable hormone levels facilitate muscle growth and maintenance.

2. Exercise methods: Middle-aged men may choose some heavy weight training more specifically to stimulate muscle growth. They usually focus on exercising large muscle groups such as the core muscles, back and legs. The exercise of these muscle groups can stimulate the body to secrete more growth hormone, thereby promoting muscle growth.

3. Eating habits: Middle-aged men may pay more attention to a healthy and nutritionally balanced diet to ensure that the body obtains sufficient protein and other nutrients to support muscle growth and repair.

4. Experience and skills: Middle-aged men may have more experience and skills and know how to promote muscle growth through training and diet. They may have gone through long-term exploration and practice and are able to use their time and resources more effectively to achieve their fitness goals.

It is important to note that muscle size and appearance are not the only measures of health. Everyone's body has unique characteristics and needs, so it's important to choose an exercise regimen that works for you and your goals.