Because MW2 uses various compression formulas, packaged textures, etc; While also programmed with everything really hardcoded, rather than having to read everything in a hierarchic method (ie: by folders and files) every time. It's also on the 360, which as arch stated, is specialized for gameplay, placing focus on graphics and memory, not hard drive space or all those extras. It also has less background progresses (nearly none really).
Arch is pretty much correct about a 1.5k poly limit, HOWEVER, if you have a really large (non-building) unit, such as a 4x4 (in a hypothetical situation: think Scarab from Military1). It can use a lot of polys. Of course, if you have a really complex building, you may use lots of polys there too. A normal unit should never really be 1.5k though because it wouldn't be apparent at the camera's zoom. This is NOT MW2, and we aren't going right up to the dude just to admire the face graphics (isn't it weird that I do that just to admire the insane work of art?)