Indeed, a one inch bigger cylinder does make the difference, and #30 appears to make 30,300 TE, making it bigger than a K28. 30 also has a little more heating surface to feed the bigger cylinders. Interesting is that although a K28 is a heavier engine (156000 vs 146000), a K28 carries much less wieght on drivers (113500 vs 12000). This would make the #30 pull like a son of a gun, but not take curves a speed too well - which I seriously doubt was a concern to the Unitah.
I would venture a guess that #30's claim to be the largest domestic narrow guage locomotive (don't know about export sales) would have stood until August 1925 when the K36's showed up.
Point to ponder - the RGS showed interest in the URY's stuff in 1939, wonder what would have happened to the #30 if the RGS would have got it?
The first thing they would have had to do is a bunch of trackwork as it had a much higher axle loading than a K27. It couldn't have run south of Rico on all that 45lb rail to Dolores (which on a good day had trouble with K27's).