Why use bigger resistors for LED?
Because your LED has an unusually high forward voltage. It is a blue or white LED, while a "normal" LED is red or green. A red or green LED has a forward voltage of roughly 2 volts and a standard forward current of about 10 mA.

Another reason is that you simply don't want to run the LED at the maximum brightness. Maybe it's annoyingly bright, and you will get a longer life out of it if you run it at a lower current.