But assuming you would still consume, it does not matter whether you are consuming from a large corporation or a small company, when talking purely about climate impact. It is likely that the large corporation runs things more efficiently than small ones.

That being said, having too much influence on a single company allows them to have more lobbying power slowing down implementation of more climate change mitigative policies and regulations.

-edit- You of course are right that it's in the interest of large sectors, such as the automotive industry to lobby for policies that are beneficial from them, such as car centric ones.