It's the same thing with the accordion. Full size accordions have 41 piano keys. The span of "adult" accordions runs from arund 18" to 20.5" for the 41 keys, measured white key to white key. My accordion keyboards are all 19.5 inches. If I play an accordion with an 18.5" keyboard I'm a mess! 20.5 inches is tolerable only because my hands are on the larger end of the scale. But there's still a TREMENDOUS difference, to the point that if I were to have to play an 18.5" accordion I couldn't do any of my speed runs at all. I could play chords and slow songs but forget about the fast ones and arpeggios.

And that's with a one-inch difference over more than 3 octaves, or 0.024" per key. I wouldn't think of buying an accordion with less than a 19.5" keyboard.

Weird thing is I also play piano (when I HAVE to) and the transition between the vastly different key sizes doesn't seem to bother me in the least. Then again, I'm a mediocre pianist at best.

Tommy