It's not explained super well, but you aren't dealing damage to the whole tank, you are trying to damage individual components.
By default your tank is equipped with APHE shells which explodes after penetrating, meaning that most penetrating hits, especially on the side, will cause the enemy tank's ammo to explode.
Alternatively you can shoot at the enemy tracks or cannon to destroy them.
I will make a better tutorial down the line, thanks for checking the game out!