Owlhoot is correct. The New Vaquero and ONE Blackhawk so far (the 50th Anniversary 357 Flattop of 2005) are the only post-1973 Ruger single actions built on a "mid size frame" similar in size and heft to the Colt SAA. This "mid-size frame" puts out it's strongest raw power when set up as a 357Mag (800ft/lbs energy in the hotter rounds) and when these guns are chambered in 45LC, cannot safely exceed about 550ft/lbs energy at MOST.
When the large-frame Ruger SAs (Old Vaquero, Blackhawk, SuperBlackhawk) are chambered in 45LC they can take "+P" loads exceeding 1,000ft/lbs energy. When chambered in 357 they're damn near indestructible
although you CAN blow them up if you really work at it with stupid handloading.
Another way to look at it: the mid-frames are a "357Mag class frame", the large are a "44Mag class frame".
Pre-1973, the standard "Blackhawk" was built on a mid-size frame, the SuperBlackhawk on the large. In 1973 they dropped the mid-frame, building everything on the large, until the mid-frame made a revival with the New Vaquero and 50th 357 Blackhawk. The NewVaq was mainly for the cowboy action shooting market as those guys don't need big horsepower and therefore wanted smaller, handier guns closer to a Colt SAA. And they got 'em, in spades.
I own a Ruger New Vaquero 357 and love it. The gun is NOT "weak", in fact the cylinder is actually beefier than a GP100's cylinder (fatter and with more metal around the bores). However, an "Old Vaquero" in 357 can be safely converted to 44Magnum (or beyond!) while my gun cannot be safely set up as a 44Magnum. (44special, sure, and some are reporting you can safely configure a NewVaq 357 as a 41Magnum...that's kinda "edgy" though...)
Fun fact: average accuracy and quality are higher on the mid-frames than on the large. When they tooled up for the mid-frames circa 2004 or so, they re-thunked how they made the guns, and the new process just works better by a smidge.