From my experience there are no way to just reduce MC's chest size, the only way to do that is not to use the black(?) poition
In areas Glittering Depths and upwards there is a vender that sell white potions . The vendor is triggered with the “It reminds you of rotten wood” option . There is also a vendor that I have only found in the Mycoid Forest that is triggered by “an unpleasant, musty odor.” He sells a pepper that “will reduce your size—everywhere”