I alternate between the convertible (Osprey Porter) and a wheeled carry-on. My wheeled carry-on weighs 5-6 pounds more than the convertible.

I reduce as much as I can, but I still can't seem to get less than 15-20 pounds of contents. The only electronics I carry are two P&S cameras. Always a book, although that's never in my 'big' carry-on. Sometimes a pair of hiking boots, and that adds about 4 pounds. Practically no toiletries.

One of these days I should pick up a small scale and actually weigh everything to see what's going on. I know, for instance, that a basic reasonably light-weight goretex shell like mine weighs a pound. You just don't think of clothing as adding up to much, but I guess it must.