Maybe it's not ideal in terms of deflecting weapons trajectories, but if the wearer has mass (be it breasts or otherwise) it needs to be accommodated somehow. Armour for men with large belly had to accommodate their physique, right? Not trying to be a smart-ass, just realistic. I don't think I've found as much to criticize about women's fantasy armour that accommodates their physique so much as I can be critical of stupid armour that leaves a lot of it exposed.
In fact womens' fantasy armour that accommodates their physique is much more dangerous to them than the stupid armour that leaves a lot of it exposed.
There is indeed armour that is built for men of ample stature. If you go to the collection of Prince Fredrick [I think] in the Hofburg in Vienna you will see a very fine example. The structure of the armour doesn't change though, it is just wider. Armour for fat men follow the same lines or structural design as armour for thin men.
Therefore the logical conclusion is that armour for a large breasted woman would look the same on the outside as armour for a small breasted women. I would just be slightly wider.
Remember also that a very thick tunic was worn under the armour, the thickness of which could be padded [at the bottom for a large breasted woman or the top for a large bellied man] in order to fill out the armour [though that is just me conjecturing]. What I noted from the armour designed for fat men is that the design did not change to facilitate the belly.
The reason that the boob plate is more dangerous than the chain-mail bikini has a lot to do with weight. Armour is a compromise between mobility and defense. Wearing armour increases your defense against blows but compromises your mobility. A woman wearing a chain mail bikini [if one was ever stupid enough to wear such a thing] would be nimble. Women wearing the sort of leather slips that are more common than the chainmail bikini would be very nimble and the best defense against a blow is not to be their when it lands.
The boobplate on the other hand has all the disadvantages of full plate armour [in its weight, even if that weight is well distributed] and in the way it effects your ability. It is not just that it is not idea in deflecting weapons trajectories it is horrific. It turns all blows from the outside in rather than from the inside out and so concentrates the force of the attacks on the very area [between the breasts and neck] that you most want to protect.
Fighting with little or no armour is a viable style if you know what you are doing. It is even more viable if you have a shield. While the chainmail bikini is indeed stupid a leather bikini or a cloth bikini would be a perfectly viable armour choice for a warrior who was practiced in a very fluid, nimble style. A boobplate is never a viable armour for anybody.
I wouldn't worry about it too much though. Fantasy armour tends to be about what is cool, not what is practical.
This for example is a very bad suit of armour.
