It depends very much on the lens, and on who built it, as well as its intended end user.
The comparison should be done with the equivalent lens from the same manufacturer to be of any use.
Compare the 400mm f/2.8 IS 1 vs 400mm f/2.8 IS II, same for the 300mm, 500mm, and 600mm.
I think that the 70-200mm f/2.8 MK II might be slightly heaver than the MK I, but not enough so to be weighing down my bag.
Canon has also produced numerous versions of the 50mm f/1.8. How does the weight compare. I've had the Japan version, the Chinese Version, I think I had a Malaysian version, and at least two US versions. I did not notice any weight creep, in fact, they are getting lighter, if anything.
The point is that you can manufacturer statistics by picking and choosing something that shows what you want to prove.