Instead of making your Mecha 6-10 men tall, would making them 1 man tall and adjusting the scenery and props to fake proper scale help with having large battlefields that don't dominate the end user hardware?
I don't think it would make a difference on the performance of the hardware? I think all it would do is give you more space to play with as the same calculations would still take place, just on a smaller scale with no overflow.
Then the texture quality has to be adjusted too. The proper solution is that the texture quality is lowered instead of the scenery/unit scaling.
Scaling geometry down doesn't require any changes to art assets, if you're scaling the entire world down it will still look the same to the user.
Smaller assets also equal a higher overall texture quality for the same resolution as the resolution of your maps are based on the real-world scale.
Assuming that he's going to position the camera in the same relative position when scaled down as when he was going to have a larger world scale (behind the mech, inside it, whatever) then the assets will look the same.