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When I was talking about all things being equal I was saying that pianos like the ones you mentioned or any series of pianos from the same maker usually have the same "basic" design other than length and price.
That is not always true. Using the Kawai upright example that Brdwyguy mentioned, the K-500 adds higher spec features beyond just height, string length, and soundboard area relative to the K-300, such as a partial duplex scale and bass agraffes. The K-800 adds a full duplex scale, a full set of agraffes (excluding the top octave as is customary) and upgraded hammer felt. One should of course focus on the overall playability and sound, and not on specs, but these differ in ways that often are associated with different product lines.
Comparing a K-300 to a K-800 is more like comparing a Yamaha U1 to a YUS5 than like comparing a U1 to a U3.