It was 35 years ago that Russia introduced its 125mm tank gun. At the time,  tank gun calibers were 90mm, 100mm, 105mm and 115mm. The Russian 125mm was a huge step forward in anti-tank weaponry, if only because it fired the heaviest anti-tank shell ever made. At 50 pounds, it was larger than the 40 pound Russian 115mm shell, the 33 pound American 105mm, 30 pound Russian 100mm or 25 pound 90mm shell. Then came the 120mm German (and soon American) tank gun. Although slightly smaller than the Russian 125mm gun, the 120mm gun was made to more exacting standards and used better designed and made ammunition. 

During the Cold War, Soviet weapons were known for poor manufacturing quality, and this reduced the penetration of the 125mm anti-tank shells. This was noticed every time the 125mm gun fought Western tanks (Lebanon in 1982, Kuwait in 1991, and Iraq in 2003). Russia has since improved the quality control and design of their 125mm gun, and produces better (at least on paper) ammunition. Countries like Israel also offer, for export, superior 125mm ammunition. But just to cover their bets, Russia is working on a 152mm tank gun. Theres no certainly that this beast will ever actually enter service. A gun that large can fire a 90 pound shell, and generate a lot of recoil. This would require a tank larger than the 60-70 ton behemoths that have proved to be about as large as it is practical for a tank to be. 

For the Russians, size did matter, but only to close the quality gap. For the decade between the introduction of the Russian 125mm gun, and the arrival of the West German 120mm gun, the 105mm gun, with excellent ammunition, was able to destroy all of the tanks equipped with the 125mm gun. But the 120mm gun enabled Russian tanks to be destroyed at more than twice as far away (over 2,000 meters). The 120mm German tank gun became one of the most successful in history, with three million rounds of ammunition produced to date.