I own a model of the VGN Triplex, and it isnt ..that big... of an engine, its trick was to get the extra tractive effort with the drivers under the tender.
It could probably fit on existing turntables of the time.
A Y6b far outsizes the triplex. For this 2-8-8-0, sure, to fit on existing turntables, but you know eventually the railroads had to up the turntables.

I would call it an incremental technical upgrade like we do with computers today, one more inch of horsepower, one more megacycle of CPU speed.