Why female breastplates don't need breast-bulges
First and most importantly, having two breast-bulges on a solid breastplate would create a concave channel just over the wearer's breastbone. Most blows that strike the breast-bulges on the inner halves (i.e. the halves facing towards the center of the chest) would naturally get deflected into this channel. This becomes a serious problem when the armor has to deal with a powerful upwards thrust; the presence of the channel would tend to direct the thrust towards the middle of the chest and up beneath whatever throat protection the wearer had at the time, whether it be a bevor or a gorget or an aventail. An experienced fighter might actually be able to see this opportunity and exploit it as soon as he/she could engage the breastplate's wearer in a close-range thrusting fight with sword or dagger.
Compare this with a conventional breastplate design where the armor's rounded surfaces are built to deflect all blows in an outward direction. The design doesn't render it absolutely impossible for a thrust to get redirected under the wearer's throat defenses, but at least the likelihood of such an unfortunate event would be much, much lower than if the wearer had a breastplate with two breast-bulges. Moreover, many conventional breastplates (such as the works of Anton Pfeffenhausser displayed here) possess a pronounced vertical keel designed to deflect the opponent's thrusts away from the centerline of the wearer's body--which also means away from the wearer's throat.
As if that wasn't enough, twin-bulged breastplates ignore the anatomical makeup of the female breast itself. To make a long story short, the breast largely consists of fat and modified sweat glands (for the production of milk, that is), and hence it's not nearly as solid as a comparable mass of muscle. So all but the largest breasts can be bound quite flat against the woman's chest without occasioning too much discomfort. In turn, this means a fighting woman probably isn't going to need a breastplate with a chest profile larger than one worn by a fighting man of a similar height and general body shape, and therefore it's quite likely that the woman would simply fit into the man's breastplate with the aid of some padding to make up the slack in the waist and shoulders.
Of course we shouldn't forget to take account of how plate armor is supposed to fit against the body. Breastplates are called "breastplates" for a very good reason: it covers the breast. The waistline on a correctly-fitted breastplate actually doesn't lie on the modern trouser waist (i.e. around the navel), but slightly higher at the bottom of the ribs (that is, the position known as the "natural waist") This marks the very bottom edge of early cuirasses like the Churburg model shown here; later breastplates with articulated faulds (such as the two 15th-century examples shown here) have an additional section below made to protect the kidneys and the hips, which extends down to approximately the height of the hipbone. This feature tends to make a male wearer's torso look a little shorter and his legs a little longer than they really are, but one one hand it's necessary to preserve the armored man's freedom of movement--a lower position for the waistline and/or bottom edge would prevent the man from bending naturally at the waist--while on the other it has the side effect of making the male breastplate perfectly wearable by women, because its high-waisted design places the inevitable chest bulge (note that I'm talking about a singular bulge, not two bulges) in a good position to accommodate the woman's breasts if she were about as tall as or slightly taller than the man for which the breastplate was designed. Further examples of male breastplates with globose chests that would have easily accommodated a woman's torso inside can be seen in this page about 16th-century Imperial armors attributed to the armorer Kolman Helmschmied and this article on the decoration of European arms and armor in the 15th and 16th centuries. Additionally, this picture displays a reproduction armor in the 16th-century Maximilian style, while this image gallery shows a high-quality replica of a 15th-century armor in action; these last two examples give particularly good illustrations of how the chest bulge on their breastplates would fit against the body of a living wearer.
A second aspect of fit that would be worth examining is how closely the armor lies against the wearer's body. High-quality harnesses of plate were obviously made to an exact fit for the customer, which would make it easy for an experienced armorer to build a female armor with just some minor tinkering upon the proportions of a perfectly ordinary male armor. And yet, by the second half of the 15th century, the state of both metallurgy and the economy in Europe had improved to such a degree that plate armor could be produced in larger quantities and at lower prices than ever before. This led to an increasing trend for ordinary soldiers (below the pay grade of men-at-arms) to wear relatively cheap munitions-grade plate armor, principally on the upper half of their bodies. These munitions-grade pieces, being mass-produced rather than individually fitted, were generally made a bit large so that all but the burliest soldiers would be able to wear them without modification; most soldiers thus found their armor somewhat oversized, but for the most part they coped easily by wearing enough padding and/or additional clothing to provide a snug fit for the armor. A woman would have been perfectly able to use the same approach. In fact, if she weren't flat-chested, she would probably have had an easier time hiding her breasts in a munitions-grade breastplate than in the largely unarmored dress of later soldiers!
In conclusion, a female warrior who wants to wear a solid breastplate isn't going to need anything more than a male breastplate sized for a man of about the same height as her. It's hard to find better proof of this idea's practical and aesthetic advantages than these suits of armor . Even medieval artists seem to have understood the principles because this 15th-century depiction of Joan of Arc shows her in male armor without any fear of contradiction!
Edited to add: I've recently been informed (thanks, Matthew Amt!) that ancient Greek and Roman muscle cuirasses actually had a similar fit to medieval European breastplates in that their "waistline" lay noticeably higher than the wearer's navel, so the anatomy depicted on the cuirass was vertically "squashed" compared to the wearer's actual musculature. The pteryges (a skirt made of leather strips) hanging from the bottom of the cuirass) might have been meant to hide this feature in addition to providing a modicum of protection to the thighs.