Dear rhino:
Here goes - I'll make a quick stab at it.
The traditional design criterion for a good tank is a careful balance of 3 basic qualities, firepower, protection and mobility. Sacrificing one quality over another tends to result in an imbalanced vehicle.
In a light tank or infantry fighting vehicle (IFV), for instance, mobility and an intermediate amount of firepower are emphasized over protection.
The result is a relatively lightweight (and thus, easily transportable), fast, quite heavily armed but lightly-armoured vehicle, unlikely to survive a hit by a heavier tank's main gun (or it's own gun for that matter) or even a large caliber automatic cannon firing Armour-Piercing ammunition.
Vehicles that were designed from the ground up to maximize all three criteria are termed main battle tanks (MBT's). Naturally the consequence of maximizing all three characteristics tends to result in a very heavy and very expensive tank.
If I had to choose the distinguishing characteristic of such AFV's (armoured fighting vehicles), it would be the heaviest, most powerful gun, mated to the heaviest armour that can be mounted. In most MBT's there is a sacrifice of mobility for these two qualities.
Historically, both weight and gun size of the MBT have been going up. Immediately after the Second world war, a tank like the T-54/55 with a 100 mm main gun, weighing about 36 tons would be considered an MBT. But nowadays that is just too light to do the job.
The Russian T-90 and the Chinese T-96 (both about 40 to 50 tons with 125 mm main gun) are about the lightest modern MBT's around. Later model Russian and Chinese tanks as well as their Western counterparts are much, much heavier up to about 60 to 70 tons, with guns ranging from 105 mm to 120 mm.
Sadly though, like the battleship, MBTs are diminishing in importance. Today everything is being downsized - including tanks. Greater mobility combined with heavy firepower is being emphasized over protection.
That is also because, (as the recent Lebanese War demonstrated) even modern heavily armoured tanks (like Israel's Merkava 1V) are vunerable to relatively lightweight anti-tank missiles.
Like the armoured knights of old, armies will give up heavy armour if it is consistently demonstrated that it cannot offer protection commensurate with the difficulty of carrying it around.
The ultimate expression of this concept is fact the flying tank, which has almost no armour to speak of, is vunerable even to small arms fire, but survives by mobility alone, and yet combines with that mobility tremendous firepower -the helicopter.
It will take another revolution in armour developement to change the present imbalance between firepower and protection.
Best Regards,
Dusky Lim
