I'll set out the reasoning why I made my comments.
Gleaned from many books over the years
The Princess was designed when there was a need to provide engines suitable for the long journey to the north with the heaviest expresses.
The King which Stanier was familiar with from his time at Swindon would be powerful enough but the firebox would be a limiting factor.
A new wide firebox design was produced but keeping some major dimensions & features from the King.
The cylinders were raised (for clearance purposes?) but kept their relative position on the frames.
The resulting loco was good but not the best that could be derived from the package.
When more locomotives were required Stanier reviews the good and bad points of the Princess
That long coupled wheelbase designed for a long boiler 4-6-0 had to go as it was of no benefit. (probably more of a hindrance?)
The GWR's dogmatic cylinder positioning and steam passage layout again provide no benefit and could be improved, by moving the cylinders forward, although a marginal increase of quarter inch diameter in size was made, the stroke remained the same.
The loco would also benefit from greater heating surfaces.
The boiler was obvious generally of a sound design as was the firebox but improvements could be made.
The boiler barrel length, tube plate distance and front ring dimensions were retained but more tube heating surface was provided.
The firebox was also improved with more heating surface, the dimensional increases of firebox and boiler are not dramatic as (AFAIK) the firebox length & height stays the same but the width is increased somewhat and provides worthwhile gains in steam raising performance. The overall increase of heating surface was in the order of 15%
Change in wheel size - No idea why - Although you explained earlier that was Riddles doing!
To me these changes suggested development of an existing design rather than a brand new clean sheet job, forgetting what had gone before.
Cheers
Keith
Edited to make clearer my observations!