Medieval Full Plates have a large empty space on the chest for weapons that pierced the plate but only a little or to lessen impacts that didn't pierce it. A kind of spaced armor. And you will be wearing gambeson underneath, which is quite bulky as well. Not to mention that the external boobs introduce weak points. Unless they are purely cosmetic and glued to the plate.
Well, yeah, but the space you're talking about usually looks like this:
That means unless you're a
granny, your boobs will still be unhappy.
And, well, later full plate wasn't really pierced
at all, or was pierced in a way that
a little extra space meant nothing. Half-swording, polearms, etc.
Also, the more space, the poorer the armour is fitting, which means it's hell to wear. So the space was mostly filled with a gambeson. Wearing
just plate was a bad idea, anyway.
All that said, I don't think we know what women wore, if anything. Most medieval art on that matter is basically fanfiction, and I think we have no surviving examples of plate armour that was known to have been worn by a woman in battle. But modern women getting their armour do want a somewhat different contour. Not boob plate (although that, too), but
different.
