For the record my (12 fret) KTS-4 is definitely louder than my (19 fret) KTC-2.
i don’t think that’s a consequence of the fretboard. I have a KTS-6 and a KTC-2, both with extended fretboards. That soprano also sounds louder than the concert, although the concert is no slouch. I think it is how they are made, and I also think it’s a matter of what frequency range is emphasized on each instrument, so subjectively, a soprano may seem louder because your ear experiences certain frequencies as loud. I have never used anything to test their objective loudness, but as they both have extended fretboards, I don’t think that the fretboard is the issue.