Unitah #30 a bit larger than a K-27. According to the folio sheet in Bender's book, it has a tractive effort of 30300 which is not correct using the traditional formula. Using it I come up with 27,300 or so. You get 30300 if you plug in 200 lbs of steam pressure. According to the folio, #30 used 180lbs. At its time of construction it would have been indeed a huge narrow gauge locomotive, bigger than the D&RG class 125's (aka K-27's).
The #30 used the same size cylinders as a K-28 (18X22), with less boiler pressure but smaller (40") drivers. A K-28 wieghs in at 156,000 verses 146,000 for #30.
There has never been anything mentioned about K-28's being the largest of anything when they were built, somewhere someithing else must have come along to claim the title.
One thing for certain, #30 was much smaller than a K-36.