what if you had a small monitor with 1440x900 that actually has a tighter dot pitch than a larger monitor with 1920x1200? how would possibly you see more detail just because its 1920x1200? it seems like it has to be the resolution in relation to the monitor size that would have to do with having more detail or crisper image.
Well, larger monitors let you sit back a little bit compared to smaller ones, even if the resolution is the same.
The reason you can see more at a higher resolution is because more pixels are used to draw the same amount of gamespace in your display area for a given object, all other things being equal. More pixels = more detail, and this makes identifying enemies and objects at a large in-game distance easier.
In other words, if you were standing in a FPS game with a very long view distance, such as BF2 (and presumably BF3 when that hits), and you looked at say a tank at 100m at 1920x1200, it would look pretty much identical to looking at a tank at 70m on a 1440x900 display, as far as detail for that object goes. Getting further into this, like I said earlier, looking at an object 300m+ away on a higher res display (2560 is amazing for this), you can clearly distinguish an enemy infantry unit crouching beside a tree or over the edge of a rooftop, whereas on the lower-res display it will more likely appear as a blob of pixels mixing in with everything else, and if it's not moving, you're going to be hard pressed to see it. This is really critical to those flying around in helis and jets looking for little ground targets as they pass over.
Even at interim spaces of 50m-100m, having the higher resolution makes it easier to target tasty hitboxes like the heads of enemy infantry, but when you really start to get closer, yeah then resolution starts to become less important. For an FPS like the olden days such as Quake1, with much less environmental detail and obstacles, and with vastly reduced distances, there wasn't a giant difference for someone playing say 1024x768 vs. 1600x1200.