It's been a while since both Tonic and Slink released their chest-reduction add-ons that allow you to further reduce the size of your breasts without quite so much distortion (pancaking), and they have been well-received by many.
How they work: Quite simply, the avatar breasts are rigged so that a given number on the avatar edit slider for breast size equals a smaller breast than it does for that avatar body normally. This means, for instance, 35 on the slider equals a smaller breast for an avatar wearing the reducer than 35 would mean without the reducer.
How they don't work (or how they might behave unexpectedly): The reason your mesh tops rigged to your specific mesh body (Tonic Fine or Slink Physique) suddenly seem too big is that they are rigged to respond to the same numbers as your regular body without the add-on - so it thinks you still have the original chest. This may not always be a bad thing if you want gap for the oversized or unhooked bra look (or for costume layering), so it's not always a bad thing. But you will need to find mesh rigged to the modified chest add-ons or use appliers/baked-on mesh tops.
It is always a good idea to try a demo before you buy these add ons, so below I've included links to the full items as well as the demos so you can save the breast for last and not make a boob out of yourself.
Tonic - Fine beauty Bust Minimizer DEMO
Tonic - Fine beauty Bust Minimizer (Three reviews, five stars)
Slink (only offered in-world, where customers can't review it)