I've never actually seen one of these external trackpads, but are they thin enough to just put inside your laptop before you close the lid? If not, that's the

I've never actually seen one of these external trackpads, but are they thin enough to just put inside your laptop before you close the lid? If not, that's the part I don't understand: if I have to carry around, and plug in, some external pointing device, I'd just bring a small mouse. But if a trackpad can fit inside the laptop, that would take less space than the mouse, and be easier to keep track of inside a backpack.