Gas turbin engine only has high effeciency in high level of working. In low level of working, the effeciency drastically decreases and fuel consumption drastically increases.
Tanks not always run on maximum speed. There are instances that they have to move at medium speed, or low speed. And Abrams's gas turbin will become a disaster.
People only use gas turbin when they hadn't manage to develop a strong diesel engine yet. T-80 gas turbin was used but didn't last for very long. Today Russia and Ukraina have de facto
abandoned gas turbin due to the fact that they all have strong diesel engine.
Armata, T-72, T-80 and T-90 diesel engine can use several kinds of fuel.
T-72, T-80, T-90, T-84, Armata engines have higher [[power : volume]] ratio than Abrams. Put 3 6TD engine together they will be as big as one Abrams gas turbin, but totally they will have 3000-4000hp compared with 1500 hp of Abrams.
Armata engine has only 1/2 the height of Abrams gas turbin. But it has 1500hp and can run with multifuel, from diesel to ethanol.
Larger but not heavier. ERA and spaced armour is created to significantly reduce the penetration power of the projectiles without significantly increase the tank's weight and armour.
And by the way I prefer ERA and spaced armour rather than DU.
The side armour of ammunition is quite thicker than top and rear, but not very thick. B41 aka RPG-7 put a hole in it.
And even if it is thick, it will cost a significant additional weight and material. Meanwhile since T-64/72/80/90 put the ammunition in the hull, below the turret, it saves up a lot of weight, and the ammo is less exposed.
Density of Vonfram is only slightly higher than uranium. But vonfram is tough, hard and heat resistant, meanwhile DU is soft and has lower melting point. When the penetrator collides with the target, the high energy of the collision can significantly increase the temperature of the penetrator, it can melt or increase the volume of the penetrator, and when the volume increases => contact size increases => pressure decreases => penetrating power decreases.
So that means, the penetrator with very high melting point provides significant advanrages. Vonfram's melting point is 3422 degree Celsius, significantly greater than uranium (1132 degree). Meanwhile Uranium carbide ceramic is heat resistant, but then the density is lesser.
The greatest advantages of DU is the price. DU is cheap and very available, while vonfram is very expensive.