Maybe most people would not think to say anything to a thin person but the particular statement was referring to a person who appears to be struggling during a workout (yes due to the story it implies they are fat but that specification was intentionally left out). It could be a pro athlete who is out on a run and is just having a bad day, it could be a lineman for a college football team out there trying to build endurance so they aren’t tired during the 4th quarter, or the parent who is just trying to get in shape. We all struggle in workouts and all need an uplifting word. It is amazing how someone saying “Keep it up!” can energize you and motivate you to push it just a little more. Everyone needs encouragement.