Really the differences which underlie the different designs is from different doctrines. The Soviets took their lessons from WW2 to heart and designed the bulk of (land) military equipment to be easy to manufacture, rugged, and with a short life expectancy (since they will be lost in battle anyway). This in contrast to Western procurement systems which incentivised arms companies to gold plate their offerings with all the latest and greatest so they can demand a higher price.
Another doctrinal difference was how the tanks would have been used. The grand contingency for both sides was an armored attack through Europe, in places like the Fulda gap, with massive tank armies of the soviet union. The Soviets designed their tanks for the attack, using a hi-low combination of T-64 (high) and T-72 (low). The T-64s were more advanced, with the best sensors and equipment the Soviets could offer, and were to spearhead the attack. The T-72 were supposed to exploit the breakthroughs made by the T-64 tank armies and wreck havoc behind the front lines of the western armies. Hence, the T-64s were designed with better protection and firepower, while the T-72s were supposed to be cheaper, numerous, and mobile. It had less protection (and emphasized side protection and protection from infantry anti-tank weapons more) because it was supposed to be confronting the opposition's non-front-line forces (logistics, bases etc).
The West designed their tanks for the defense. Hence the massive frontal armor and massive turrets. The tanks were to sit in prepared entrenched positions and pick-off the soviet armor as it advanced, and then retreat to the next line of prepared positions, to deny the Soviets the breakthrough they were seeking. The tanks didn't have to be as numerous, since they had only a few strategic choke points to guard, but they had to be individually difficult to destroy. They didn't have to be as strategically mobile, hence mass was not as big of an issue.
Attached is a 3-view blowup of a T-72BM
You can see the hull front armor is substantial and composite. The hull side armor at the vertical part is the standard 80mm. At the angled part it is thinner.
---------- Post added at 09:13 PM ---------- Previous post was at 09:07 PM ----------
Really, until the M1 appeared, the Soviets did not have worse tanks. The T-62,64,72 were clearly superior to the M-60. The M1 and the T-80 (its contemporary) compared quite closely. Only with the M1A1 and later did the US have an arguable advantage over Soviet Union/Russia.
