Documentation [1] says:
The small_model option configures a separate model for lightweight tasks like title generation. By default, OpenCode tries to use a cheaper model if one is available from your provider, otherwise it falls back to your main model.
I would expect that if you set a local model it would just use the same model. Or if for example you set GPT as main model, it would use something else from OpenAI. I see no mentions of Grok as default