I agree with Robb Barnes: that it is the large frame, same as the Super Blackhawk (and old model large frame).
All the Ruger Flat Top Blackhawks including the .44 Special are built on the old model mid-size frame except the .41 and .44 Mag New Model Flat Top Blackhawks which are large frames.
The Ruger mid-size frames are larger than the Colt New Frontier as well as the Colt SAAs. And the Ruger mid-size cyls are ~1.675" diameter, ~.020" larger than the Colt cyls.
USFA cyls are also~.020" larger diameter than all the Colt cyls, with the exception of the USFA "Old Armory Original" model, intro'd around 2009 which is true to the original Colt cyl dimensions of ~1.655".
USFA also sold 'Old Armory Original Model' Colt replacement cylinders of 1st, 2nd and 3rd gen dimensions which are what I've used to replace my 2nd and 3rd gen Colt cylinders because USFA cyls have authentic 1st gen size flutes, and black powder chamfers at the front end of the flutes.
So Ruger and the standard USFA with ~.020" larger cyls and frames are a bit stronger than Colt's.