F1 used to issue numbers to the teams rather than the drivers - two per team, with each team having two identical cars. Over the years, the numbers got smaller as the advertising space became more valuable. At one point it was pretty much only possible to tell the difference between team mates by their helmets. Then, it was decided to let drivers choose a permanent number to build a brand around (e.g. Lewis Hamilton has Team 44), and to increase the visibility of the numbers. The latest cars have a number on each side near the rear wing and another on the nose.
Obviously, F1 cars don't have doors, so the numbers will never be as big as NASCAR. It'll still be strange seeing NASCAR with no large door numbers. Imagine the General with no numbers on the doors - it's just wrong!