I don't know...
It wasn't as conservative a time as you may think. Yes, more conservative than today, but she is wearing a corset, which was meant to slim the waist line and raise the breasts. Even small breasted women would appear to be more buxom just by the nature of the garment, as the design will show you below. (It's also pretty close to what Liz is wearing in the trailer)
It was the style around 1912 to wear these corsets. If you look at the picture below, it shows a woman of the era wearing a corset. The photo is a little white washed, but if there was dynamic lighting, you better believe she'd be sporting a mighty cleavage.
If you were a stylish, attractive young woman, like Elizabeth, in 1912, you would most likely be wearing these type of corsets which would give her the appearance she has. Also, the waist coat and dress look appropriate to the time as well. I do think her hair maybe a bit moddish for 1912. It would probably be more extravagant and pinned up, or she'd be wearing a hat. Not quite sure on that.