I have updated my rig to use breast physics. This way the breast get animated by physics so the movement is more 'realistic' and more in line to the motion of the rest of the body.
As a test I worked on the DoubleBed Reverse Cowgirl Femdom Stage 2 animation.
In order to allow your sims breasts to move you will need the mod by Denton47 found here:
http://www.loverslab.com/topic/81008-sims-4-breast-bones-unlocked-for-animation-final-v3/
This new way of animating the breast bones allows me to make adjustments from one animation to another in order to make their movement more realistic. I will of course continue to work on it to make as more accurate as I can. The good thing however is that now I can update previous animations to use physics, much easier than redoing them.
Try it ingame with your sims and please tell me what you think about the movement.
I have also tested it with very small sized breasts. Since the movement is recorded with physics, the smaller the breasts, the smaller the movements. I have not tested other peoples animations with small breasts however to see if the same thing happens for them too. I would love to know however, if you have made any similar test.