Dunno – it was a repeat being shown on one of those high numbered channels on freeview.
Did a pretty basic experiment (a repeat of an older one apparently) where they fed a group of slim/normal students 5000 Kcal a day for several weeks and monitored how their bodies changed.
All gained weight but to different extents and in different ways… some stored it in muscle others in fat.
Some peoples metabolism will adjust to meet the calorie intake in their diet. Some peoples bodies will omit chemical signals to prevent them over eating in the first place.
It is thought that much of this is dictated by your genes and your childhood development etc etc.. and that for the average adult your weight will be pretty constant bar a small amount of middle aged spread.
Essentially… embrace that belly.