Breast reduction surgery reduces the amount of breast tissue and skin. Therefore, it usually only affects the cup size (C to B) and does not usually change the chest size (36 to 34).
but make sure your surgeon can do your procedure with different techniques as a vertical reduction may be adequate over the traditional anchor reduction (more scars) as there are some surgeons (believe it or not) that only do reductions one way and one way only. I call them dinosaurs.