I've looked at a cloth from 3d-Age, and I've remarked that if you uncheck "Animated" on the Body's parameter, you may change your character's breasts shape without influencing the corresponding part of the cloth.
Here's an example with Carly (believe me: she's somewhat "generous" with her assets at this place )

As reference: the cloth "volume" here was based on Carly's assets. I unchecked "Animated" and I could reduce Carly's breasts (BreastLarge = -0.200 above, = 0 below)
Originally:

Note that actually I don't do animation, even though I would like to learn (missing time)
Oh and my laziness blocked me when I looked for the word "animated" in Poser's documentation, sorry about that 

𝒫𝒽𝓎𝓁

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