Yes... the boobie issue! Well, I am definitely not boobie, so it still lays nice on me and the necklace stays in the long "U" formation.
I would agree with you since you are boobie. 20 motifs (and other long necklaces) are tough for boobie ladies as they tend to sit in the valley, trapped between the mountains!

It's similar to a Magic long pendant where I've seen some Magic long necklaces land. The necklace ends up in a "Y" formation, stuck in the valley.
The problem is length...
What would look great is the 16 motif. It's longer so it will land in a better position, past the boobies and further down the torso.
To help visualize... think of the boobie ladies in the Gilded Age and their gorgeous long diamond necklaces and pearl strands that went past their torso. So gorgeous! I love that look.
You could try the 20 motif, but my guess is, sure, it will work, but it won't look super great, especially when you start moving.
Try on the 16 motif too. It's also a bit more airy if you like that.