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In the case of Kawai K series uprights, even the tonal style varies. I think that is less true of Kawai grands at least within the GX and SK series. The tonal style of Boston uprights also seemed more uniform, at least with the 126 and 132 models (much moreso than with the K-500 and K-800).
Robert Estrin demonstrated the tonal consistency of a Baldwin/Hamilton studio upright and Baldwin 7' grand in this video:
These pianos clearly have very different designs, but both were engineered to deliver Baldwin's tonal signature at the time.
Short of confirmation from Kawai, I have no way to verify this, but it currently is my speculative belief that Kawai designs the K-500 and ST-1 to have a little heavier action and warmer tone than the K-300 or K-800 have so that they will require less voicing and regulation maintenance in heavy use institutional settings.
The upshot of all of that is that pianos of different length have different scale designs. That the same woods, design and build methods are used may contribute to tonal consistency, but ultimately, I think if there is a consistency of tonal style across a product line, it is engineered in as a design goal.