Hint: It's not because peaches contain more water than apples.
Peaches contain a bit more water than apples (88 percent and 84 percent, respectively), but that's not why peaches shed so much more juice during cooking. It's because peaches have very little pectin, while apples have it in abundance. When apples are cooked, they release water, but the pectin in their cell walls absorbs most of it instead of allowing it to leak out. Because peaches have far less pectin, they don't retain their juice nearly as well.