Why smaller is faster?
Regarding your Q2, keep in mind that this is in the context of logic gates, which in CMOS logic are switched by driving the gate of a MOSFET.  MOSFET gates behave like small capacitors.  To change the state of these gates, i.e. to charge or discharge the capacitor, you need to apply a current to it.  The time it takes to charge or discharge will depend on the magnitude of the current applied, so more power gets you faster switching.