NKP -vs- C&O cylinder size

As information, pretty much ALL modern steam locomotive cylinders AND valves have liners pressed into the main casting. Thus, when the cylinder/valve bores become too worn, the old liners are split and removed, and new liners for the cylinders, or new cages for the valves, are installed. The new components are then bored to original specifications.

Enlarging the cylinders could over-stress the running gear machinery too. 

i would think just as big of a factor is the extra 19 tons the K4 has over the drive wheels.

Seems there's a lot about the C&O machine that was tweaked out to give the most "stuff" they could pull out of the basic package.  This wound up being one awesomely versatile design.  IIRC, the origin for this whole exercise was the Erie "S" series, which was tested by AMC and then morphed into a stretch job for C&O.  The resulting T1 2-10-4 was good enough to inspire a re-shrink job resulting in Berks for NKP and PM....and eventually C&O.  The Texas, of course, gave rise to  PRR's best steamer ever, although I'll admit that PRR would have found more uses for an AMC Berk, without having to reshuffle the railroad.  So you figure out what you want on it, and then pay the man.   Just like Pizza !