F-stop

I have a P&S Canon sx100is. its lenses are 6-60mm (i think that it is 30-300 on 35mm), and on the lens is said that the aperature is 2.8-4.3. I plan to by d3100 with a kit lens 18-55 (27-82.5 on 35mm) and the aperature on that lens is 3.5-5.6.

There is a few things i just cannon understand about aperature. I hardly belive that my sx100is can have larger aperature than 18-55 nikon. I understand that it is f/2.8, so what i want to know: on wide open canon f/2.8 if the focal lenght is 6mm does it mean that diameter of pupil will be 6/2.8 = 2.14mm, or do i need to calculate its 35mm focal lenght, 30/2.8= 10.7mm? But that means that on 300 it will be 300/4.3= 69mm, which is imposible. I really need some explanation on that.
Thanks.