Throughout history the balance between heavy armor and light but fast movement has been instrumental in determining the victors in many battles. Some modern examples:
In the early 20th century, the argument was made that speed is armor when designing warships. The HMS Hood was an example. Battleship sized guns were placed on a ship with cruiser armor. Because a cruiser did not have as much armor, this enabled the ship to have a much higher turn of speed. As you probably know, when the Hood ran into the German battleship Bismarck, the heavier armored ship literally blew the battlecruiser out of the water. Another example was the Soviet T34.
On June 22, 1941 Germany invaded the Soviet Union. The main battle tank for the German army at that time was the Panzer III. What the Germans did not know was that the Soviets had developed the most advanced tank in the world, the T34. The T34 was heavier, carried a much larger weapon, and had better range. The T34 was actually faster in adverse conditions because of its much wider tracks. It did not become stuck as easily. The Germans built the Tiger tank to fight the T34. As you also probably already know, the Tiger was the heaviest tank in the world when it arrived on the battlefield. This did not translate into the same type of success as with the T34. The Tiger had limited range (Its fuel economy was low) it was slow and prone to breaking down. True, it was a tough tank to destroy in battle. This was also offset because it was difficult and expensive to build. In other words, they could not make nearly as many. The German Panther tank was also developed to fight the T34. This tank was lighter than the Tiger and carried a smaller main gun. This was offset by the fact that it was much faster, had much better range, was easier to tow away, was also easier and less expensive to build. Production of the Panther began almost a full year after the Tiger, yet more than 4 times as many were built before the end of the war. Even though the Panther was lighter than the Tiger tank, it was almost as difficult to destroy in battle.
The overall point is that as a general rule, balance is key. (As in so many other aspects in life) This is one area where I personally had disagreement with former Secretary of Defense Donald Rumsfeld. Future wars may demand a lighter, more agile force. It is almost a certainty that some type of heavy weapon system(s) will be useful, if not a vital part of those wars.