It's larger than the switch! And the case for the Steamdeck is notably thicker than the Steamdeck, while the case for the switch is not. I think this is because the switch is almost flat, whereas the joysticks on the front of the Steamdeck and the handholds on the back stick out a lot farther, and the Steamdeck case doesn't conform to the Steamdeck; it's just as large as the largest dimension in any direction.
My experience is that chucking the Steamdeck in my backpack for a flight is doable if I don't put it in the case, but if I put it in the case it takes up enough space that it's annoying to fit my laptop / other stuff too.
I normally deal with this by either not using the case or just taking my switch instead. But I respect the author's choice here a whole lot :)
Okay then I guess I must've gotten that one wrong too, since I always imagined Switch is slightly larger than GameBoy Advance or PSP.
(They really need to start adding bananas for scale for to ads for those devices. Or I need to watch more ads; the few photos I saw gave me the mental picture of a PSP-sized handheld.)