... not Aces High II, with its on-line site carrying the bulk of the load and requiring only 53.53 MB.
According to XP Add/Delete Program sizes, and of course speaking about only programs on my computer:
Tied for 5th and 6th largest are IL-2 FB and IL-2 AEP with 1,657 MB each (which could make IL-2 2nd at 3314 MB considering combined size -- not sure if both files really that large, but that's how XP lists them).
4th largest, including 11.18 Server Manager, is America's Army at 1953.18 MB.
3rd largest is Far Cry at 3,070 MB.
2nd largest is MS Flight Sim 2004 at 3,282 MB.
And the winner, largest of all by the whopping margin of nearly 14 GIGabytes, is ...
X-Plane 8.0 at 17,189 MB!