Energy balance

In healthy young men, overfeeding by ~1,000 kcal/day for 21 days does not trigger a significant adaptive increase in energy expenditure (other than obligatory thermic effect), resulting in 85-90% of excess energy being stored as fat and protein.

If you are a healthy young man, eating 1,000 extra calories a day for three weeks will not significantly boost your metabolism to burn off those calories. Instead, your body will store about 85-90% of that excess energy as fat and protein. You might naturally eat less later, but during the overfeeding period, you will gain weight because your energy expenditure does not increase enough to match the intake.