The reason they went to 73 has nothing to do with weight, but with the current fad of retro keyboards. Look onstage with so many current chart bands, and you'll see Rhodes's, Wurlitzer's, vintage synths, Mellotron's, you name it. It's kind of like so many guitarists' obsession with vintage guitars and amps (so few of them can play them well enough for the difference to be noticeable). It's all about the LOOK...

I think Nord started the trend, bringing out their Electro series in a 61 and a 73. Perhaps understandable in this particular case, as a) the keyboard wasn't splittable, and b) it ONLY emulated B3 and Rhodes/Clav/Wurli's. The main size of Rhodes's was 73. More would have detracted from the emulation.

They became incredibly popular, the retro trend gathered momentum, and the next thing you know, to be 'cool', a modern keyboard needs to ape an older, one-trick-pony electro-mechanical piano!

It is the keyboard equivalent of bell-bottom trousers. A fashion with no practical need, followed by legions of kids desperate to look 'hip', no matter how ridiculous it is going to look later on.