I'd like to 'love' your comment

Hanbok have the concept that one should look roughly like an upward pointing triangle. (major point at the top two points at the bottom) This is because the original Hindu symbol of an upward pointing triangle roughly means "Heaven". This was brought into Buddhism (Since Hinduism and Buddhism have similar roots) and by the time of the drama. Sunny Yang's Hanbok book also says that the upper class women had more petticoats to emphasize this fact. So an equilateral triangle, apparently was more desirable, which also happens to fit the original Hindu symbol. (So if you were a commoner, you'd literally be seeing the lords/yangban as "Heaven" by the clothes they wear--that's got to have psychological value.)

In order to achieve that, the breasts are made to look flat, which also goes with the Confucianism idea of a modest and chaste woman--no temptation for the men. Which is why the lower class women tie their chima (the skirt portion) under their breasts. It doesn't matter to them.

Because of the wide skirt and petticoats, it means that the woman looks like she's floating/gliding. (As supposed to the Japanese kimono... you can't run in that thing... I tried. It's more modesty through elegance. So woman is deliberate in her steps. She's conscious of where she is going. She is elegant in the Japanese model.)

This was particularly emphasized in the Joseon era... when Neo-Confucianism was at its height.

However, this is set in the Silla Dynasty. (Formal Korean doesn't have sh sounds, BTW--unless you talk lazy. It's along the lines of the "Nugu/Dugu" kinda thing.) That was pre-unification of Korea, BTW. In that case, the influence of the clothes was mostly from Han China (202-220). And the flattened breast wasn't such a big thing. Objectively you will see breasts in pre-Joseon especially with the ties at the waistline, especially on the commoner class.

Seondeok was 606-647, which means Buddhism was in place by then since the dates for Buddhism arriving into Korea date around 200-400 CE. (there is debate, so I'm adjusting for it).

Also some Asian women don't have large breasts. =P Surprise the men so they discover they do have them at all? (not to get too lewd). Kimono also favors that, making the neckline the "erotic" part (Lisa Dalby's Kimono)

Anyway, I suspect it's mostly modesty. I do have to say wearing hanbok is really easy as a woman. Super comfy. Most men hate their traditional garb--which is always amusing to me. (My brother even refuses to wear the hat--I was tempted to buy it for him as a gag present, but it was too expensive as a gag present, especially the ones made of the traditional horsehair). That's a reverse of most men's clothes. Plus, I find it funnier, since it's usually the men determine the dress code in male-dominated societies... so the idea of Joseon men volunteering to feel uncomfortable is funny to me.