It's not the size so much as the complexity. If you have a scene with lots of trees/buildings the computer not only has to draw them but it has to figure out which bits not to draw because they're hidden - from your point of view. Then comes the shadows. On top of that you overlay several dozen figures which move and also have to be oriented properly. There are several cheats one can use, but believe me, rendering 3D scenes in real time isn't easy, which is why 3D graphics cards have their own computers built-in. Lower end machines would find it difficult to cope, frame rate could plummet if several dozen troops ran across your path into some trees.
I am aware of how 3D graphics are rendered, but I think you are assuming that these new troops would be some super-high count polygon ultra real models (I SEE ACNE!!!).
If you are running AHII right now, trees, tanks, planes, buildings, and all (even at lowest settings), your computer is probably going to handle 10 or 20 troops with a higher polygon count than we currently have. I imagine if new troops made a computer chug, then dogfights must make it cry.