Researchers have identified a system that 'senses' nutrient flux (flux = what's going in versus what's going out) through both fat and muscle cells. This affects a lot of process. When dieting, more calories are leaving the fat cell than are going in (negative flux). This nutrient sensing system 'senses' this and affects many processes, one of which is leptin production (decreasing leptin production).
So leptin drops.
When overfeeding, more calories are entering the fat cells than are leaving (positive flux). The system 'senses' this and affects many processes, one of which is leptin production (increasing leptin production).
So leptin increases.