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I agree with Carsten's assessment on the DO. When I was looking for a zoom in this range for my dad, I compared it with the 70-300 IS, which is longer but, IIRC, it was either the same weight or lighter than the DO. I preferred everything about the 70-300 IS, particularly the performance at 300mm and the price, which was less than half of the DO.
You could consider the Pentax 300/4. I've never shot with it, but Jim B has one and it's tiny. Not quite as short as the DO, but I bet it weighs less.
70-300 IS L or non L? i've been looking at the pentax, seems they made a number of different versions, and i've been having trouble finding samples from the smc A, which is the small one i believe. for me weight isn't a concern, it's just space that i'm short on. i wish somebody would make collapsible prime telephotos, they really are ridiculously long and full of empty space when you get to 300mm and above. i'm not really interested in the shorter parts of the focal range on these zooms, so they do feel like a bit of a waste of optical correction to some degree.