Schematic (from here):
Why are four legs connected to the green LED and two to the amber LED in the 4017 traffic light circuit? Shouldn't it be able to do with only one leg per LED?
Have a look at the LED truth table on the same page:
Each output is consecutively active for the same time. By wired-OR-ing outputs you can create longer active times. So the green LED will be on during 4 time units.
Normally you would have to wired-OR Q0 through Q4 for the red LED, but we have a carry output (here called ":10") which does just that, see the bottom trace of the timing diagram: