I don't believe that the Welte mechanism itself is actually that heavy.
It's just that they weren't installed in inexpensive, relatively
lightweight pianos.

I once helped move a straight 1913 Mason & Hamlin upright that was
heavier than _any_ other upright I'd ever encountered.  And my 1914 Red
Welte Mason & Hamlin didn't seem to weigh that much more.

Just the nature of the beast.

Bob Hodge