Two factors stand out as a major difference between the two classes...
The tender loaded on the C-21 was about 1,300lbs heavier. Was this an over-estimate on capacity needed for a loco this size, or did one of the classes just get what tender was available?
The C-21 had 111 2 1/4" tubes, while the C-25 had 199 2" tubes. The math puts the effective area at about 30% more on the C-25. Firebox numbers are almost 50% larger on 375. The weight on the drivers was about 25% more, and the tractive effort was about 20% more. Is there any science to explain how the lowest increase was the TE?