Imagine, if you will, a tank. It's a full-on beast: wide base, massive treads, two enormous cannons, independently rotating turret, surface details including antennas and whirling radar dishes, you name it. As it rumbles powerfully along the 3D landscape it leaves twin tread marks in the sand. Smaller vehicles will skitter around it, powersliding across the terrain, but the tank is a rock: it moves with weight and power. Next, imagine taking your mousewheel and spinning it, slowly moving the camera out. Your tank is not alone; dozens more are continually spewing from a factory, rolling out to join the first. Most real-time strategy games stop there, but not Supreme Commander. As you keep panning out, a shoreline becomes visible, and just offshore sits a battleship.
The battleship is enormous. Compared to the tank, the side of the hull is a wall of steel. The huge ship doesn't fit on a single screen, no, you have to keep zooming out to take it all in. The deck of this monstrosity is surrounded by independently-rotating anti-aircraft guns, each one bigger than the tank. Mighty columns of steel rotating on gigantic turrets signify the battleship's main bombardment guns: the screen shakes when they fire, and they can hurl giant shells as big as a car for miles inland. Compared to the battleship, the tank is a tiny little bundle of pixels. But wait... you can keep scrolling.
The scale in Supreme Commander is so immense, your units may sometimes step on your other units. Each of these little guys is as big as a tank, and the enomrous spider is bigger than a city block.
As you pan back, you see that the battleship is not alone. It's surrounded by a flotilla of cruisers and destroyers, surrounding aircraft carriers in formation and leaving rippling white wakes in the blue sea. Subs drift menacingly along the outskirts of the fleet, only their conning towers breaking the surface of the ocean. At this scale, the curving shore of the main landmass is visible, and the tanks are so small that they are now represented by icons. No more than dots, really. But this mass of arms on the coast is just a tiny part of the action happening around the globe. As you keep panning out, even your enormous fleets become little more than symbols on the map, representing your enormous ground, naval, and air forces. At this level you can give commands to your fleets, your army, your air force: "fly here, attack here, coordinate to invade this landmass simultaneously..." War is fought on a massive scale. You're not just futzing with little squads of men or tanks: You are the Supreme Commander.