Well it is also burning oxygen from air which you don't carry on the helicopter.

Let me see how much high school chemistry I still remember...

    CO2 = C (12) * 1 + O (16) * 2

So 1 lb of C from fuel => 3.67 lb of CO2 out

Suppose AvGas is just Octane, C8H18

    C8H18 = C (12) * 8 + H (1) * 18

1 lb of octane => 0.84 lb of C => 3.09 lb of CO2

So yeah, you get 3 lb CO2 from 1 lb of octane.