I remember reading a book (i think it was called the Tank encyclopedia was around half an A5 (in length) and around 200 pages with a hard cover), that detailed how armour on tanks began to change to counter the change in AT weaponry around the 60s-70s. From what i understand reactive armour and layered armour became dominant while larger single shelled tanks less so.
At the end of WWII, the caliber needed for effective AT guns had gone up from easily handled guns in 37-40mm range at the start of the war, to monstrosities like the PaK 43 88mm. Such large weapons were extremely difficult to maneuver by hand, needing around a dozen men to move it short distances under ideal conditions.
http://www.ww2incolor.com/d/452600-4/PAK+43++88mm_
Because AT missiles/rockets rely on shaped charges rather than kinetic energy in the case of AT guns (for the most part, some did use HEAT rounds), the power (and as a result, the mechanism size) needed for launching the projectile was greatly reduced while still increasing the penetration ability of the round. The result is man-portable weapons that can knock out a tank, eliminating the need for a towed artillery piece.
This process of conversion was cemented by the advent of guided AT missiles in the 60s, allowing high levels of accuracy at long ranges and at moving targets. Now something as small as a Jeep fitted with a guided missile, can knock out any tank on the battlefield at ranges exceeding that of tank guns.