While Delta Force doesn't have state of the art graphics, it compensates for this by having immense battlegrounds with rolling terrain and a clear view for 100 km away. It accomplishes this by using NovaLogic's Voxel Space 3 technology. This is a game that was made, in my opinion, for long-range combat. If NovaLogic wanted to make another typical close-range FPS, they would have made small maps in which sniper rifles were useless. But in this game, snipers rule with the long-distance view.
Polygons are ideals for precise and extremely detailed images, but only for a small visual area.
That's the reason why all these games generally take place inside buildings or within an enclosed area. You just can't render enough polygons fast enough to recreate the image of a building that is in a valley, three kilometers away with any precision. This is where the Voxel, short for volume pixel, technology comes in. As you might know, a pixel is one of those small squares that composes a small fraction of your screen. And while using this technique reduces the quality of close-up images, like soldiers or buildings, it allows you to see them clearly from a long distance.