This is where a nations "philosophy" of what "Armour" is comes into play. What order do they set their priorities on "Protection", "Manoeuvre", and "Firepower". We see with the British the priorities were on Protection, Firepower and then Manoeuvre in their Centurion, Chieftan, Conqueror, and Challenger. The Germans with the Leopards placed an empathise on Manoeuvre, Firepower and Protection. The Soviets put a empathise on Firepower, Protection and Manoeuvre. Where the West was looking at High Tech gunnery and ergonomics for crews, the Soviets were looking at basic/crude mechanics that would withstand a Conscript military, and gave no thought to crew comfort or fatigue. They made their tanks small in size to offer smaller targets to their enemy. They produced their AFVs in "volume" with the philosophy of having overwhelming odds in their favour and fighting aggressively. As was mentioned, the West placed more value in their "highly trained" crews and looked at giving them better protection and "working conditions" which meant larger, more complex AFVs.
So you can see that the West and the Soviets had two completely different philosophies on AFVs.