Well, as size goes I have the 28-300 L zoom with my canon cameras. It is HEAVY and BIG but in gigs when I don´t know what is waiting for me - it is a very usable lens.Now I have the Nex 5 with the 18-200mm lens, which is equals a 27-300 in full frame cameras. If you ever had this two setups side by side you´ll know which one is smaller, easier to carry around and still does the job wonderfully.But to each one size is different.
PS. I bought the Nex 5 and 18-200 combo because I nearly paid for the lens and the price of the camera was only 9 euros :)