I'm just thinking of any kind of real life scenario where this would make sense somehow. A tank that is good against other heavies but not good vs weak. The only thing that came to my mind would be some kind of a heavier cannon that slowed down the tank because of weight. But usually there are no upgrades that nerf a unit.
Other than that we could simply say the cannon gets so strong, that the bullet will just fly in medium tanks and leave it on the other side without exploding. Haha.
Don't try to find any IRL sense of it, it is only for the sake of balance