I listened to the second demo, here the Kronos piano sounds much, much better. To me both Nord and Kronos pianos are on the same level here.

Regarding the difference of 200 MB to 4 GB, I found that most of the Gigabytes are spent for the long decay of the tones, pedal resonance, many velocity layers, but the main "substance" of the piano sound can be achieved with less than 200 MB. When the Krome came out, I recorded its 2,8 GB German Grand (identical to Kronos) as well as the 128 MB expansion piano of the Korg M3 - they sound nearly identical. Really surprising, but on the other hand it's clear, e.g.if you have 16 velocity layers instead of 4, it's 4 times the data amount. Make 21-second-long samples instead of looping after 3 seconds, and you get 7 times the data amount. Summed up, it's 4 x 7 = 28 times the data amount - but the piano timbre as such isn't different at all, the first 3 seconds of the piano sample at a certain velocity sound absolutely identical.