I believe that for (1), the tabs overlap so that they take the same amount of space as the old tabs. It's an illusion that they're larger (which I initially fell for also, until I was corrected).
(2) is the same as Chrome and older Firefox versions - to ensure that there is still some draggable area in the window when it is filled with tabs, and it disappears when the window is maximised.
(2) is the same as Chrome and older Firefox versions - to ensure that there is still some draggable area in the window when it is filled with tabs, and it disappears when the window is maximised.