If we are talking logic, I would say that a large creature should take less damage than a medium one.
Say the flames are 5 feet high, they would cover a medium creature up to the shoulders, while a large creature standing on 4 sqaures gets one leg partly covered in flames. I would say that if we follow logic, lighting all 4 squares at once should be needed to do full damage... Even that should do less damage, as the larger creature still is less covered in flames compared to a medium one.