The most important parameters of the tank body are armor and hit points (HP). There are 2 kinds of armor: kinetic and thermal.
Kinetic armor protects tanks from "physical" damage (ie from machineguns, cannons, rockets, etc.). Thermal armor is used to protect the tank body from "heat" damage type weapons such as flamers, lasers, thermite bombs, etc.
Note: to produce tanks with medium bodies you need build a factory module. Tanks with heavy bodies require 2 factory modules.