I've found that, for mobile screens, the most valuable refinement real estate is often the top row of the results. Since that space is very limited, there's only room for the most useful facets or filters. And you have to decide whether to show keys or values, e.g., "Brand" as a key or "Nike", "Adidas", etc. as values. Showing keys takes up less space and allows you to cover more ground, but showing values may be more useful -- and certainly more discoverable -- to the user, since there's one less step. As with all things, it's a tradeoff, and I don't think there's been that much research on optimizing it.