Shortest code for longest output
Python: 8 chars code, 387420489 chars output -- Ratio: 48427561.125 : 1

'a'*9**9


We can have the ratio tend to infinity by adding more **9s:

'a'*9**9**9
'a'*9**9**9**9
etc.


For example:

'a'*9**9**9**9**9**9


which has a ratio of ~10101010108.568 (unimaginably large number).