In a way it is quiet a sensible move, although I don't understand the 5cm reduction. It's not going to make a real difference, but just cause hassle.
My cabin bag is usually small, even smaller than the Ryanair allowance. However, I often have a laptop - this is very difficult to get inside the small bag. My cabin bag + laptop side by side would easily fit into any airline baggage cage (inc RYR), but for all LoCo airlines this isn't acceptable, which really is a nonsense.