You can get LED's with different "brightnesses".
Also the amount of light coming from a LED can be limited if you use a current limiting resistor that is too large (you MUST have one or the LED will fry).
Also some of the Arduino boards have a built in current limiting resistor on pin 13, if i remember correct. If you hook a LED with another resistor up to pin 13 you will haqve 2 current limiting resistors, which will reduce the brightness.