Update: There were a few years around 2000 (not sure which) when Yamaha's sound priorities differed from mine. However, things have been relatively straightforward with the 5X5, 6X5 and 7X5 series, as the first number represented the order of introduction and the centre number represented the position in the range. Each generation incorporates specification upgrades at every level, but (at least as far as sound and touch are concerned) those at the lower end of the range see more nominal upgrades than those at the top end.
For example, the 525, 625 and 725 have "GH3", "GH3X" and "GHS" weightings respectively.
In terms of sound modeling, Virtual Resonance Modelling is introduced to the 635 and later the 725 (joining the higher end of the range).
In terms of available piano sounds, the 525 has CFIIIS piano but the 625 has CFX wirg binaural sampling and Bosendorfer Imperial (becoming more like the rest of the range in this regard).
There are acoustic/touch changes at the top of the range that do not show up in the specs, including better padding of the the key movements in the top end of the 7X5 series.
The power supplies at the top end of the range are now external, but I don't know if this is for acoustic reasons or just to make the individual parts lighter.
Naturally, many things have remained unchanged; for example, ounterweights have yet to trickle down from the X85 end into any of the other instruments.
Based on Yamaha's own specs, e.g. https://europe.yamaha.com/en/products/musical_instruments/pianos/clavinova/clp-775/specs.html