The trick is that you want it to seem to be reflecting things at its own scale, not from the real world. If it mirrors large objects it will ruin the scale effect. It’s why painting things a single plain color tends to flatten a miniature out – it’s not big enough to generate its own realistic shade and shadow, so you have to paint it to exaggerate the effect and make it look “right.” The same is doubly true for shiny metal. If it has light and dark areas that seem like they’re reflecting things at the mini’s own size, it will maintain the overall illusion better.
Interesting notion. I’ve never thought of it that way before.