If he states that you're good you can bet that you are, even if the commander has the final responsibility his head will roll too in the case of any incident related to insufficient de-icing. No two situations/conditions are the same and any comparison between two de-icing cases is moot.
What you should have been doing, if you felt insecure about if the de-icing was properly conducted, was getting out of your seat and checking it for yourself. Part of our job is taking other professionals words for granted, it is something we have to do as we have neither the skill nor the time to check every aspect or detail of a flight. However, it is our responsibility to check everything that we suspect may be wrong, either by ourselves or by seeking a second opinion from a professional.
If you felt the need to ask here if the amount of fluid was enough (post flight!!!) I can only assume that you didn't check, otherwise you would have known, and took off anyway. Now that's something that one really could have opinions about....
Edit: saw that you weren't part of the crew - sorry.