I have long been in the practice of packing (or just buying) a reusable shopping bag to use as carry-on luggage. When empty, at least the kind that lays flat when empty, they take up almost no space at all. If you really want to protect items, you can enclose everything using two bags (put one inside the other upside down.)
Another trick is to bring/pack/use giant ziplock bags. The super (above 2.5gallon) ones are expensive, but the 2 gallon/2.5 gallon ones (if you can find them) are not expensive, and they hold quite a bit. (though the bigger ones fit more per bag) On your way down, you wouldn't need to use them, but on your way back, you can kinda use them like space-saver bags. The seal-top kind work better for this purpose than the 'slider' style ones. If you over pack them the zipper breaks, but they can help compact anything that is at all fluffy. (winter hoodies, stuffed animals, pillows). they are also useful if you have something messy like a wet bathing suit.