Substantive: having substance
Substantial: having large quantity of substance
Substantial is a more specific adjective than substantive. In other words, something that is substantial implies that it is substantive. The converse is not necessarily true. For example, if you witnessed a minor car accident, the scratch on the car was substantive. The damage actually happened and it was real. You did not imagine it. However, the damage was not substantial because it was just a minor scratch.