Mega Spawn are the result of a colony of immature Seed Spawn.
All Mega Spawn that are fought in game consist of a Colony of Immature Seed Spawn, so they are not very large, and are often cumbersome, and waddle. They will however travel quickly with the shadows and on walls and cielings. In Total Assault 3 they will crawl above the player and drop down from the cieling onto them. They stand around 7 feet tall in game.
Mega Spawn are intensily hard to kill, and are only really vulnerable to heat based weapons and explosives. They are not generally fought in the first game, not face to face any way, and are only encountered 4 times in Total Assault 2. Although they are rare, when they are fought, you better come prepared.