It may well not be anywhere near as simple as "double the armor, double the protection" or "diminishing returns". Remember, armor amounts have to deal with different star levels on different strata. That is, it needs to deal with 5* armor on depth 1 and 1* armor on depth 20, and it needs to do it in a way that 'makes sense". Protection may not be a percentage of damage done (e.g. damage reduced by 27%) and it may not be absolute (e.g. damage reduced by 2.7 bars).

For weapons, it has been measured that, say, a 2* sword does less damage in T1, but it also does less damage in T3. Likewise, it may well be that if you only have a little protection from a certain kind of damage that doubling the amount of armor more than doubles your protection, but after a certain point, more armor gives you very little extra protection, and it may all depend on what level you are on. Or, it may be if you have only a small amount of protection and get hit hard, you get effectively zero defense, much like a leather jacket won't protect you from a bazooka. Or, it may be some other system.