Yes, I have the smartypants MS-20 Mini edition for extra special needs… derp. 
Seriously, you might be right, though two things speak against this:
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To keep prices low it would make a lot more sense for Korg to use both keybeds in both products.
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although the original MS-20 did indeed have shorter keys, thus the same proportions are being reproduced in the MS-20 Mini’s keybed, the angle at which they are being photographed, as well as the perception when you look at it from the front can be a bit misleading.
A quick mockup… I had to (proportionally) distort the MS-20 pic as a whole a bit to match the angle of the Arp, so this doesn’t proof anything except, that the keybed’s proportions are the same.
Also the 45° cut on the white keys can be seen in both models, so at least I expect the Arp’s keys to be the same width as the MS20.