Duffels and backpacks are the weight-savers of the cabin world: typically 0.8 to 1.6 kg empty, against 2.8 kg or more for a hard trolley. They also compress, which matters when a locker is nearly full or a bag is a centimetre over a printed size. For multi-stop trips, students and anyone travelling on a strict 7 kg fare, this is usually the smartest category.
Check the stated dimensions stuffed, not empty — a backpack packed round can outgrow the sizer even if its label size is fine. Packing method is covered in how to pack a hand carry bag.
