I think MarshmallowPB might not actually support scaling the breasts with a slider. I've tried many times and found that it seems to only work with the body's blendshapes, but the PhysBones generated by Marshmallow don't follow the slider changes themselves. The official docs also state that due to how the system works, resizing in VRChat isn't possible. Maybe you could try entering Play Mode and manually recording a scale-up/down animation as a workaround? Though you'd probably need to bake the physics parameters for each size, which could be quite a bit of work.
