Your point isn't exaclty valid, Kid_Vid. Whenever a new transformers series started, they always reworked the characters and then stuck in Optimus Prime and Megatron to fit the setting. Megatron was a gun in the original (and if an 80' robot can become a small hand gun, then why can't a cassette tape become a 7' tall robot?) then he was melted down and turened into Galvatron in the movie and futuristic series that followed it. In that incarnation he was a huge cannon. In Beast Wars he was a T-Rex whose head became his main cannon and in Energon he was a massive tank.
So let's no mince words here, he has always been a a character with a huge main weapon. Turning him into a jet is just stupid. How many jets do you see with a massive main gun? None. Well, maybe the A-10, but you can only fire the main gun for 3 seconds or it slows the plane down. The point being, if htey had to rework the transformers to adhere to the Law of Conservation of Mass, then why not a tank? A plane needs to be streamlined, and one with a cannon like Megatron usually has in just not feasible.
Gabriel