This is just really funny to me if you don’t know what else is going on. Even though Time Lords probably don’t gain weight.
The Doctor is just really concerned about his recent winter weight gain and even though Grace warned him to not overdue it with the Christmas cookies she offers to help him. Grace puts him on a proper diet and they go on adventures that include necessary running. By the New Year he has lost twenty pounds. Grace doesn’t think twenty pounds in a week is possible but the Doctor just scuffs…superior Time Lord physiology.