Professional "repackers" like FitGirl Repacks can sometimes compress games by up to 50% without losing quality.
The safest way to play Modern Warfare 2 (2009) is through official digital storefronts or verified physical copies. Call of Duty®: Modern Warfare® 2 (2009) on Steam
Below is a generic, safe procedure using a FitGirl-style repack as an example.