I was watching Dark of the Moon yesterday. It bothers me that the primary antagonist is not that strong.
In the first film he was stronger than ever; he transformed into a Cybertronian Jet. In the second film, he was resurrected into a new form of Cybertronian tank.
Why did he transform into a Mack truck?
According to the video game, it was a target of opportunity to escape the Decepticon base, which was being invaded by the Autobots.
To escape the base you must go through multiple fan tunnels, spawn many hatchlings to do your bidding, and kill the Autobots. When going through the tunnels, you come across a truck which is Megatron's alternate form in TF3. Megatron then scans the truck and takes its form. Now you can drive through out the area.
