Chemistry occurs at the surface of the food - if you have lumps of peaches in your stomach then they have to be digested, ie broken down. If you drink a smoothie then the surface area available to chemical reactions is a lot larger, so digestion happens quicker. Quicker digestion of sugars causes a larger insulin spike, which is bad. Also commercial smoothies usually have a lot more sugar and fat in them than you'd think, I know you said just blended peaches, but in reality everyone adds stuff to it.