What would you say to someone who believes their actions are too small to make a difference for the environment?

Progress on environmental issues depends on the scale at which actions take place. It is not so much about the efforts of an individual, but about the collective effort. Even a small change at the societal level leads to better environmental conditions compared to nothing, or even isolated cases of ecoactivism.

They have a good argument to be honest. While you should take steps to decrease consumption for the betterment of the environment, regular citizens do not (as of currently) make up the bulk of the damage.

No action is too small it might be in the moment but in the long run, it might get bigger.

I would say that kind of thinking starts to affect everyone and then no changes are made, so every action is important.

If we all thought like that then progress would cease to exist. Baby steps are greater than a crawl and eventually we will be able to see a larger change. People want to fit in and do the popular thing, so slowly getting people on board will make the biggest difference.