If I trim a few hairs near my breast would they grow thicker or more?

Or just pluck them. New hair growth after cutting may seem thicker & stiffer. That's because the "cut end" of the hair feels coarser. The previous hair ends that were older (and softer & thinner by being "worn down" by friction of day-to-day life) have been cut away. The part of the hair that is outside the hair follicle is not alive, so cutting hair does not change what is happening in the hair follicle in the skin.