More to do with weight rather than fat/thin
Depends on the temperature of the room - are they trying to keep warm or cold ? :)
Impact of weight on calorie burn :
https://www.livestrong.com/article/314404-how-many-calories-do-you-lose-per-mile/
Not just diet - it's combination of diet and exercise...........if you take on more calories than you burn, the excess will be stored as fat - quite simple really at that level. Metabolism tends to slow as you age too (so we need to eat less and/or move more, basically)
This is quite...