But in FreeSpace, enemies can sneak from behind and break havoc. Therefore, invasions call for a different approach. The D-Day was successful mostly because the Allies gained air superiority over Normandy, and the Luftwaffe couldn't sink all those warships and troop transports well before they could reach the land. Had Hitler decided to deploy aircraft, the Allied retreat from Dunkerque would have been a carnage. No threats from the air = more chances of success.
I'd say that ground troops are deployed progressively (massive formations of transports are too vulnerable to strikes), but I still don't know how many transports are needed to deliver all those soldiers.