I'm not 100% sure, but I find that aiming for "critical" areas of the tanks makes a substantial difference. The gap where the turret meets the body, the rear, underbelly and treads (not the armor side skirts) make the most impact. With the direct rear and underbelly being the "squishiest" bits. Everywhere else and you're hitting what should be the reactive armor, so less damage. Also having the Engineer's armor damaging perk helps a lot too, though that only comes into play a little later in the matches. I've sat in a tank for most of a match and never got into the red (without constant repairs) and I've been in matches where I get 1-2 rockets or an AP tank shell in the back side and I'm completely cooked even with 2-3 people repairing.